BMC Medical Research Methodology

Article	Citations
Mind the gap: covariate constrained randomisation can protect against substantial power loss in parallel cluster randomised trials	409
Does imbalance in chest X-ray datasets produce biased deep learning approaches for COVID-19 screening?	404
coreSCD: multi-stakeholder consensus on core outcomes for sickle cell disease clinical trials	299
regCOVID: Tracking publications of registered COVID-19 studies	246
A gated group sequential design for seamless Phase II/III trial with subpopulation selection	179
Comparing statistical methods in assessing the prognostic effect of biomarker variability on time-to-event clinical outcomes	170
Understanding facilitators of research participation among adults with self-reported chronic pain – a survey examining hypothetical research participation	159
Correction: In health research publications, the number of authors is strongly associated with collective self-citations but less so with citations by others	119
Pre-statistical harmonization of behavioral instruments across eight surveys and trials	117
More than one third of clinical practice guidelines on low back pain overlap in AGREE II appraisals. Research wasted?	102
Should RECOVERY have used response adaptive randomisation? Evidence from a simulation study	101
Recruitment methods and yield rates in a clinical trial of physical exercise for older adults with hypertension—HAEL Study: a study within a trial	89
How likely is unmeasured confounding to explain meta-analysis-derived associations between alcohol, other substances, and mood-related conditions with HIV risk behaviors?	88
Handling of missing component information for common composite score outcomes used in axial spondyloarthritis research when complete-case analysis is unbiased	87
External validation of existing dementia prediction models on observational health data	83
Statistical methods in the analysis of multicentre HIV randomized controlled trials in the African region: a scoping review	82
Choices of measures of association affect the visualisation and composition of the multimorbidity networks	71
Modelling of intensive care unit (ICU) length of stay as a quality measure: a problematic exercise	70
A Bayesian network perspective on neonatal pneumonia in pregnant women with diabetes mellitus	67
Developing survey weights to ensure representativeness in a national, matched cohort study: results from the children and young people with Long Covid (CLoCk) study	65
MetaAnalyst: a user-friendly tool for metagenomic biomarker detection and phenotype classification	58
Design and analysis of outcomes following SARS-CoV-2 infection in veterans	57
The impact of iterative removal of low-information cluster-period cells from a stepped wedge design	56
Power and sample size calculation for incremental net benefit in cost effectiveness analyses with applications to trials conducted by the Canadian Cancer Trials Group	56
A framework to model global, regional, and national estimates of intimate partner violence	56

Contextual effects: how to, and how not to, quantify them	55
Technologies for frailty, comorbidity, and multimorbidity in older adults: a systematic review of research designs	54
Estimation of marginal structural models under irregular visits and unmeasured confounder: calibrated inverse probability weights	54
CalScope: methodology and lessons learned for conducting a remote statewide SARS-CoV-2 seroprevalence study in California using an at-home dried blood spot collection kit and online survey	52
Clinical systematic reviews – a brief overview	51
A data-driven pipeline to extract potential adverse drug reactions through prescription, procedures and medical diagnoses analysis: application to a cohort study of 2,010 patients taking hydroxychloro	50
Causal effect of chemotherapy received dose intensity on survival outcome: a retrospective study in osteosarcoma	49
Open science saves lives: lessons from the COVID-19 pandemic	45
Using group testing in a two-phase epidemiologic design to identify the effects of a large number of antibody reactions on disease risk	45
Methodological aspects for accelerometer-based assessment of physical activity in heart failure and health	45
An algorithm to assess importance of predictors in systematic reviews of prediction models: a case study with simulations	44
Joint modelling of multivariate longitudinal clinical laboratory safety outcomes, concomitant medication and clinical adverse events: application to artemisinin-based treatment during pregnancy clinic	44
The effect of high prevalence of missing data on estimation of the coefficients of a logistic regression model when using multiple imputation	42
Analytical challenges in estimating the effect of exposures that are bounded by follow-up time: experiences from the Blood Stream Infection—Focus on Outcomes study	42
The reliance on conceptual frameworks in qualitative research – a way forward	42
Assessing and adjusting for bias in ecological analysis using multiple sample datasets	41
A study within a trial (SWAT) of clinical trial feasibility and barriers to recruitment in the United Kingdom – the CapaCiTY programme experience	41
Effect of alcohol consumption on breast cancer: probabilistic bias analysis for adjustment of exposure misclassification bias and confounders	40
A systematic review of methods to estimate colorectal cancer incidence using population-based cancer registries	40
Study retention and attrition in a longitudinal cohort study including patient-reported outcomes, fieldwork and biobank samples: results of the Netherlands quality of life and Biomedical cohort study	39
Methodology and experiences of rapid advice guideline development for children with COVID-19: responding to the COVID-19 outbreak quickly and efficiently	38
Measuring the impact of zero-cases studies in evidence synthesis practice using the harms index and benefits index (Hi-Bi)	37
Bayesian spatial-temporal analysis and determinants of cardiovascular diseases in Tanzania mainland	37
Improving retention of community-recruited participants in HIV prevention research through Saturday household visits; findings from the HPTN 071 (PopART) study in South Africa	36
Fidelity, pragmatism and the “grey line” in between—exploring the delivery of a pragmatic physical activity randomised controlled trial—a secondary analysis	34
Measuring bereavement prevalence in a complex sampling survey: the 2019 Georgia Behavioral Risk Factor Surveillance System (BRFSS)	34
Discretizing multiple continuous predictors with U-shaped relationships with lnOR: introducing the recursive gradient scanning method in clinical and epidemiological research	34
Optimising research investment by simulating and evaluating monitoring strategies to inform a trial: a simulation of liver fibrosis monitoring	33
Comparison of two propensity score-based methods for balancing covariates: the overlap weighting and fine stratification methods in real-world claims data	33
Using joint models to study the association between CD4 count and the risk of death in TB/HIV data	33
Evaluating methods to define place of residence in Canadian administrative data and the impact on observed associations with all-cause mortality in type 2 diabetes	32
Blinded sample size re-estimation in a comparative diagnostic accuracy study	31
Bayesian additional evidence for decision making under small sample uncertainty	31
Using web conferencing to engage Aboriginal and Torres Strait Islander young people in research: a feasibility study	31
Development and validation of paired MEDLINE and Embase search filters for cost-utility studies	30
Identifying Potential Factors Associated with High HIV viral load in KwaZulu-Natal, South Africa using Multiple Correspondence Analysis and Random Forest Analysis	30
Cause of death coding in asthma	30
Binary classification with fuzzy logistic regression under class imbalance and complete separation in clinical studies	29
Machine learning computational tools to assist the performance of systematic reviews: A mapping review	29
Predicting postoperative surgical site infection with administrative data: a random forests algorithm	29
Target trial emulation with multi-state model analysis to assess treatment effectiveness using clinical COVID-19 data	29
Misspecification of confounder-exposure and confounder-outcome associations leads to bias in effect estimates	29
Reporting of methods to prepare, pilot and perform data extraction in systematic reviews: analysis of a sample of 152 Cochrane and non-Cochrane reviews	29
A photograph of the researcher on the invitation letter did not affect the participation rate of a postal survey: a randomized study within a trial (SWAT)	29
Comparison of analysis methods and design choices for treatment-by-period interaction in unidirectional switch designs: a simulation study	28
Genetic matching for time-dependent treatments: a longitudinal extension and simulation study	27
The potential impact fraction of population weight reduction scenarios on non-communicable diseases in Belgium: application of the g-computation approach	26
Inadequate reporting quality of registered genome editing trials: an observational study	26
Accounting for bias due to outcome data missing not at random: comparison and illustration of two approaches to probabilistic bias analysis: a simulation study	25
Development and evaluation of a search filter to identify prognostic factor studies in Ovid MEDLINE	24

Standardizing registry data to the OMOP Common Data Model: experience from three pulmonary hypertension databases	24
Feasibility of a hybrid clinical trial for respiratory virus detection in toddlers during the influenza season	24
Development of a clinical tool for rating the body function categories of the ICF generic-30/rehabilitation set in Japanese rehabilitation practice and examination of its interrater reliability	24
Implications of analysing time-to-event outcomes as binary in meta-analysis: empirical evidence from the Cochrane Database of Systematic Reviews	24
Prevalence of evidence of inconsistency and its association with network structural characteristics in 201 published networks of interventions	23
Practical basket design for binary outcomes with control of family-wise error rate	23
Adjusting expected deaths for mortality displacement during the COVID-19 pandemic: a model based counterfactual approach at the level of individuals	23
TRIPLE C reporting principles for case study evaluations of the role of context in complex interventions	23
Bayesian variable selection and survival modeling: assessing the Most important comorbidities that impact lung and colorectal cancer survival in Spain	23
Estimation and inference for the mediation effect in a time-varying mediation model	22
Correction to: Applying a novel approach to scoping review incorporating artificial intelligence: mapping the natural history of gonorrhoea	22
Correction: Individual patient data to allow a more elaborated comparison of trial results with real-world outcomes from second-line immunotherapy in NSCLC	22
Comparison of generic and specific instruments to assess the quality of life in patients with melasma	20
A validation study of data in the National Tonsil Surgery Register in Sweden: high agreement with medical records ensures that data can be used to monitor clinical practices and outcomes	20
MSMpred: interactive modelling and prediction of individual evolution via multistate models	20
Opportunities and obstacles in linking large health care registries: the primary secondary cancer care registry - breast cancer	20
A PubMed search filter for efficiently retrieving exercise training studies	20
SurvdigitizeR: an algorithm for automated survival curve digitization	20
Documenting patients’ and providers’ preferences when proposing a randomized controlled trial: a qualitative exploration	20
Evaluating methods for Lasso selective inference in biomedical research: a comparative simulation study	20
A new likelihood model for analyses of pharmacoepidemiologic case–control studies which avoids decision rules for determining latent exposure status	20
Interrater reliability estimators tested against true interrater reliabilities	19
Delays in reporting and publishing trial results during pandemics: cross sectional analysis of 2009 H1N1, 2014 Ebola, and 2016 Zika clinical trials	19
Using random-forest multiple imputation to address bias of self-reported anthropometric measures, hypertension and hypercholesterolemia in the Belgian health interview survey	19
Comparison of methods for tuning machine learning model hyper-parameters: with application to predicting high-need high-cost health care users	19
Structured expert elicitation to inform long-term survival extrapolations using alternative parametric distributions: a case study of CAR T therapy for relapsed/ refractory multiple myeloma	19
Comparison of the effects of imputation methods for missing data in predictive modelling of cohort study datasets	19
Application of machine learning in predicting survival outcomes involving real-world data: a scoping review	19
A comparison of survival models for prediction of eight-year revision risk following total knee and hip arthroplasty	19
A roadmap to using randomization in clinical trials	19
Development of a standardized patient-reported clinical questionnaire for children with spinal pain	18
Development and validation of a model to identify polycystic ovary syndrome in the French national administrative health database	18
Elucidating vaccine efficacy using a correlate of protection, demographics, and logistic regression	18
Locating and testing the healthy context paradox: examples from the INCLUSIVE trial	18
Comparison of Pocock and Simon’s covariate-adaptive randomization procedures in clinical trials	18
Stacked survival models for residual lifetime data	18
Challenges in measurement of adolescent mental health: how are gender patterns affected when level of symptoms is analysed simultaneously with impairment?	17
A flexible approach for variable selection in large-scale healthcare database studies with missing covariate and outcome data	17
Data-driven prediction of COVID-19 cases in Germany for decision making	17
The methodological quality of systematic reviews regarding the Core Outcome Set (COS) development	17
Adjusting for Berkson error in exposure in ordinary and conditional logistic regression and in Poisson regression	17
Correction: A scoping review of statistical methods in studies of biomarker-related treatment heterogeneity for breast cancer	17
Predicting kidney graft function and failure among kidney transplant recipients	17
SpatialWavePredict: a tutorial-based primer and toolbox for forecasting growth trajectories using the ensemble spatial wave sub-epidemic modeling framework	17
Does the feedback of blood results in observational studies influence response and consent? A randomised study of the Understanding Society Innovation Panel	17
Text analysis framework for identifying mutations among non-small cell lung cancer patients from laboratory data	16
Evaluating the performance of propensity score matching based approaches in individual patient data meta-analysis	16
Estimating causal effects in the presence of competing events using regression standardisation with the Stata command standsurv	16
Evaluation of intervention systematic reviews on chronic non-communicable diseases and lifestyle risk factors in low-middle income countries: meta-research	16
Analysing cluster randomised controlled trials using GLMM, GEE1, GEE2, and QIF: results from four case studies	16
Exploring flexible polynomial regression as a method to align routine clinical outcomes with daily data capture through remote technologies	16
Recruiting and retaining community-based participants in a COVID-19 longitudinal cohort and social networks study: lessons from Victoria, Australia	16
Association of ABO blood groups with venous thrombosis recurrence in middle-aged patients: insights from a weighted Cox analysis dedicated to ambispective design	16
Symptom clusters among cancer survivors: what can machine learning techniques tell us?	15
Selecting a randomization method for a multi-center clinical trial with stochastic recruitment considerations	15
Concordance between humans and GPT-4 in appraising the methodological quality of case reports and case series using the Murad tool	15
CAT HPPR: a critical appraisal tool to assess the quality of systematic, rapid, and scoping reviews investigating interventions in health promotion and prevention	15
A scoping review on the methodological and reporting quality of scoping reviews in China	15
The difference between concealment and blinding in clinical trials and why both are important. A reply to Garg and Mickenautsch, BMC Medical Research Methodology (2022) 22:17	15
Determining diagnosis date of diabetes using structured electronic health record (EHR) data: the SEARCH for diabetes in youth study	15
Piloting an automated clinical trial eligibility surveillance and provider alert system based on artificial intelligence and standard data models	15
Prescription Sequence Symmetry Analysis (PSSA) to assess prescribing cascades: a step-by-step guide	15
Stroke survivors’ preferences regarding study participation in rehabilitation research	15
A new method for testing reproducibility in systematic reviews was developed, but needs more testing	15
Correction to: Applied comparison of large-scale propensity score matching and cardinality matching for causal inference in observational research	15
Individual-specific networks for prediction modelling – A scoping review of methods	15
Network analysis for estimating standardization trends in genomics using MEDLINE	14
Multi-arm multi-stage (MAMS) randomised selection designs: impact of treatment selection rules on the operating characteristics	14
Systematic comparison of approaches to analyze clustered competing risks data	14
Development of the multivariate administrative data cystectomy model and its impact on misclassification bias	14
Inconsistencies in self-reported diabetes in a large panel study: the Survey of Health, Ageing and Retirement in Europe (SHARE)	14
Bias amplification in the g-computation algorithm for time-varying treatments: a case study of industry payments and prescription of opioid products	14
Sample size recalculation in three-stage clinical trials and its evaluation	14
Development and validation of a distributed representation model of Japanese high-dimensional administrative claims data for clinical epidemiology studies	13
Multi-group global tests for restricted mean survival time and restricted mean time lost: a variable transformation approach	13
Estimating reference intervals from an IPD meta-analysis using quantile regression	13
Ranking of treatments in network meta-analysis: incorporating minimally important differences	13
Adaptation and psychometric assessment of a sexual and reproductive empowerment scale in Arabic among refugee and non-refugee adolescent girls	13
Reporting of flow diagrams in randomised controlled trials published in periodontology and implantology: a survey	13
Comparing single and multiple imputation strategies for harmonizing substance use data across HIV-related cohort studies	13

Agreement test of P value versus Bayes factor for sample means comparison: analysis of articles from the Angle Orthodontist journal	13
Prospective sampling bias in COVID-19 recruitment methods: experimental evidence from a national randomized survey testing recruitment materials	13
The relationship of publication language, study population, risk of bias, and treatment effects in acupuncture related systematic reviews: a meta-epidemiologic study	13
The impact of heterogeneity on the analysis of platform trials with normally distributed outcomes	13
Methodological perspectives on the study of the health effects of unemployment – reviewing the mode of unemployment, the statistical analysis method and the role of confounding factors	13
Correction: CRIME‑Q—a unifying tool for critical appraisal of methodological (technical) quality, quality of reporting and risk of bias in animal research	13
A scoping review of Q-methodology in healthcare research	12
Machine learning is an effective method to predict the 90-day prognosis of patients with transient ischemic attack and minor stroke	12
The effect of dichotomization of skewed adjustment covariates in the analysis of clinical trials	12
Analysis of applying a patient safety taxonomy to patient and clinician-reported incident reports during the COVID-19 pandemic: a mixed methods study	12
Correction: Sampling strategies to evaluate the prognostic value of a new biomarker on a time-to-event end-point	12
Correction: Feasibility of a hybrid clinical trial for respiratory virus detection in toddlers during the influenza season	12
Multiply robust estimator for the difference in survival functions using pseudo-observations	12
Methodological guidance for the evaluation and updating of clinical prediction models: a systematic review	12
Automated medical chart review for breast cancer outcomes research: a novel natural language processing extraction system	12
Evaluation of data imputation strategies in complex, deeply-phenotyped data sets: the case of the EU-AIMS Longitudinal European Autism Project	12
A logical analysis of null hypothesis significance testing using popular terminology	12
Longitudinal plasmode algorithms to evaluate statistical methods in realistic scenarios: an illustration applied to occupational epidemiology	12
Correction: Impact of sampling and data collection methods on maternity survey response: a randomised controlled trial of paper and push-to-web surveys and a concurrent social media survey	12
Modeling of atrophy size trajectories: variable transformation, prediction and age-of-onset estimation	12
Comparison of Bayesian Networks, G-estimation and linear models to estimate causal treatment effects in aggregated N-of-1 trials with carry-over effects	12
Grey literature scoping review: a synthesis of the application of participatory methodologies in underrepresented groups at an elevated risk of dementia	12
The gap between statistical and clinical significance: time to pay attention to clinical relevance in patient-reported outcome measures of insomnia	12
Protocol implementation during the COVID-19 pandemic: experiences from a randomized trial of stress ulcer prophylaxis	12
Some examples of privacy-preserving sharing of COVID-19 pandemic data with statistical utility evaluation	12
Correction: Evaluating quasi-experimental approaches for estimating epidemiological efficacy of non-randomised field trials: applications in Wolbachia interventions for dengue	12
High-dimensional mediation analysis for continuous outcome with confounders using overlap weighting method in observational epigenetic study	12
Development of an Aboriginal Resilience and Recovery Questionnaire – a collaboration between practitioners and help-seeking clients of a Victorian Aboriginal community controlled health service	12
Negative log-binomial model with optimal robust variance to estimate the prevalence ratio, in cross-sectional population studies	11
Penalized landmark supermodels (penLM) for dynamic prediction for time-to-event outcomes in high-dimensional data	11
Sensitivity analyses for data missing at random versus missing not at random using latent growth modelling: a practical guide for randomised controlled trials	11
The classification algorithms to support the management of the patient with femur fracture	11
Borrowing information across patient subgroups in clinical trials, with application to a paediatric trial	11
Methodological approaches to measuring the incidence of unplanned emergency department presentations by cancer patients receiving systemic anti-cancer therapy: a systematic review	11
Accommodating heterogeneous missing data patterns for prostate cancer risk prediction	11
Patient-centered infertility questionnaire for female clients (PCIQ-F): part I: questionnaire development	11
Bayesian dynamic borrowing in group-sequential design for medical device studies	11
Multimorbidity prevalence and health outcome prediction: assessing the impact of lookback periods, disease count, and definition criteria in health administrative data at the population-based level	11
Count data models for outpatient health services utilisation	11
Advancing guideline quality through country-wide and regional quality assessment of CPGs using AGREE: a scoping review	11
Determining the extent and frequency of on-site monitoring: a bayesian risk-based approach	11
Converting from face-to-face to postal follow-up and its effects on participant retention, response rates and errors: lessons from the EQUAL study in the UK	10
Propensity score analysis for health care disparities: a deweighting approach	10
Measuring advance care planning behavior in Dutch adults: translation, cultural adaptation and validation of the Advance Care Planning Engagement Survey	10
Handling missing values in patient-reported outcome data in the presence of intercurrent events	10
A comparison of frequentist and Bayesian approaches to the Personalised Randomised Controlled Trial (PRACTical)—design and analysis considerations	10
Ensuring cross-cultural data comparability by means of anchoring vignettes in heterogeneous refugee samples	10
Patient regional index: a new way to rank clinical specialties based on outpatient clinics big data	10
Addressing treatment switching in the ALTA-1L trial with g-methods: exploring the impact of model specification	10
Identification of patients’ smoking status using an explainable AI approach: a Danish electronic health records case study	10
How important is the linearity assumption in a sample size calculation for a randomised controlled trial where treatment is anticipated to affect a rate of change?	10
Estimating effects of health policy interventions using interrupted time-series analyses: a simulation study	10
Performance metrics for models designed to predict treatment effect	10
Grouped data with survey revision	10
Predicting lung cancer survival prognosis based on the conditional survival bayesian network	10
Advancements in predicting and modeling rare event outcomes for enhanced decision-making	10
Design and statistical analysis reporting among interrupted time series studies in drug utilization research: a cross-sectional survey	10
The minimal important difference of patient-reported outcome measures related to female urinary incontinence: a systematic review	10
Evidence pointing toward invalidity of the SF-8 physical and mental scales: a fusion validity assessment	10
nlive: an R package to facilitate the application of the sigmoidal and random changepoint mixed models	10
Left-censored recurrent event analysis in epidemiological studies: a proposal for when the number of previous episodes is unknown	10
Comparing outcomes from tailored meta-analysis with outcomes from a setting specific test accuracy study using routine data of faecal calprotectin testing for inflammatory bowel disease	10
A compartmental model for smoking dynamics in Italy: a pipeline for inference, validation, and forecasting under hypothetical scenarios	10
Evaluation of the Fill-it-up-design to use historical control data in randomized clinical trials with two arm parallel group design	10
Sample size calculation for prevalence studies using Scalex and ScalaR calculators	10
From simple to even simpler, but not too simple: a head-to-head comparison of the Better-Worse and Drop-Down methods for measuring patient health status	9
Two-step spatiotemporal anomaly detection corrected for lag reporting time with application to real-time dengue surveillance in Thailand	9
Understanding multimorbidity: insights with graphical models	9
Adjusting for outcome risk factors in immigrant datasets: total or direct effects?	9
BivRec: an R package for the nonparametric and semiparametric analysis of bivariate alternating recurrent events	9
The optimal approach for retrieving systematic reviews was achieved when searching MEDLINE and Epistemonikos in addition to reference checking: a methodological validation study	9
Robustness assessment of regressions using cluster analysis typologies: a bootstrap procedure with application in state sequence analysis	9
External control arm analysis: an evaluation of propensity score approaches, G-computation, and doubly debiased machine learning	9
Standard of care for COVID-19 in randomized clinical trials registered in trial registries and published in preprint servers and scholarly journals: a cross-sectional study	9
The validity, reliability and feasibility of four instruments for assessing the consciousness of stroke patients in a neurological intensive care unit compared	9
Longitudinal mediation analysis with multilevel and latent growth models: a separable effects causal approach	9
Recruitment into antibody prevalence studies: a randomized trial of postcards vs. letters as invitations	9
Bayesian Mendelian randomization with an interval causal null hypothesis: ternary decision rules and loss function calibration	9
A new infodemiological approach through Google Trends: longitudinal analysis of COVID-19 scientific and infodemic names in Italy	9
Cluster randomised trials with a binary outcome and a small number of clusters: comparison of individual and cluster level analysis method	9
Should multiple imputation be stratified by exposure group when estimating causal effects via outcome regression in observational studies?	9
Streamlining systematic reviews with large language models using prompt engineering and retrieval augmented generation	9
Major mistakes or errors in the use of trial sequential analysis in systematic reviews or meta-analyses – the METSA systematic review	9
Are morbidity and mortality estimates from randomized controlled trials externally valid? A comparison of outcomes among infants enrolled into an RCT or a cohort study in Botswana	9
CRIME-Q—a unifying tool for critical appraisal of methodological (technical) quality, quality of reporting and risk of bias in animal research	9
Interchangeability and optimization of heart rate methods for estimating oxygen uptake in ergometer cycling, level treadmill walking and running	9
The conclusiveness of trial sequential analysis varies with estimation of between-study variance: a case study	9
Adaptations for remote research work: a modified web-push strategy compared to a mail-only strategy for administering a survey of healthcare experiences	9
Dementia risk prediction in individuals with mild cognitive impairment: a comparison of Cox regression and machine learning models	9
Tailored approach to participant recruitment and retention to maximize health equity in pediatric cancer research	9
Constructing within and between hospital physician social networks for modeling physician research participation	9
A systematic review and psychometric evaluation of resilience measurement scales for people living with dementia and their carers	8
Case-only approach applied in environmental epidemiology: 2 examples of interaction effect using the US National Health and Nutrition Examination Survey (NHANES) datasets	8
Exploring data reduction strategies in the analysis of continuous pressure imaging technology	8
Model-based standardization using multiple imputation	8