Presentation on theme: "Non-randomised studies Methodologies for a new era summer school School of Applied Social Studies, University College Cork 22 June 2011 Dr Paul Montgomery."— Presentation transcript:
Non-randomised studies Methodologies for a new era summer school School of Applied Social Studies, University College Cork 22 June 2011 Dr Paul Montgomery Jennifer Burton
Why bother? The data from a good study can be analysed in many ways, but no amount of clever analysis can compensate for the problems with the design of a study. (Altman, 1991)
Introduction It is sometimes impossible or undesirable to influence events in a human sample l You may not be able to control group allocation l It may be unethical to expose or withhold an intervention
An appreciation of the varieties of study designs available can reduce the need to reinvent the wheel and to rediscover the mistakes of others. It can also help to end the “scandal of research”’ (Altman, 1993)
Aims Identify types of questions that can be answered using non-randomised methods Describe several non-randomised designs Highlight the strengths and weaknesses of common non-randomised designs
Question to design Prevalence/ incidence Risk and protective factors Prognosis Harm Effectiveness
Nature, prevalence of social problem Risk, protective factors Intervention Trials: Efficacy Systematic Reviews and Meta-Analyses Intervention Trials : Effectiveness Practice Guidelines Evidence Based Practice: Judicious application of research to individual clients and organisations
Internally Valid Evidence of Effectiveness Meta-Analysis of Randomised Trials Randomised Trials Non-randomised studies Cohort studies Case-Control studies Non-comparative studies Case Series (open trial) Case reports Expert opinion The Hierarchy of Evidence For Intervention Studies Illustrative examples, hypothesis generating studies
Question to Design In practice, methods tend to be complementary in answering questions Each method may be used to answer several types of questions Several studies may help tease apart a question
Prevalence How many people have mental health disorders?
Surveys Census (one-time full population) l Ideal but expensive, difficult, likely to miss people from marginal groups Cross section (one-time sample) l Must consider many potential biases due to geography, time, etc.
Cross-Sectional Survey Identify a sample of adults representative of the population Measure symptoms of mental health problems Calculate the number of people above a given threshold
Surveys Usefully estimate prevalence or incidence and associations Comparisons may be made between different subgroups to identify associations
Risk and Protection What factors can predict falls in the elderly?
Retrospective Cohort Take all elderly people in Oxford Look at l Characteristics of their homes l Individual factors (e.g. medication use) l Other predictors? Look for association between these factors and falls
Retrospective Cohort Backward looking survey Relatively inexpensive and practical Good for detecting latent outcomes Prone to several sources of bias (selection, participant recall, etc.)
Risk and Protection What are the factors that contribute to chronic fatigue syndrome?
Prospective Cohort Take all babies born in a given period in 1970 Survey them regularly Look for correlations between variables (e.g. maternal depression) and outcomes (e.g. chronic fatigue)
Prospective Cohort Identifies temporal relationships Can examine multiple effects of exposure Loss-to-follow-up can be a problem Inefficient for the evaluation of rare problems unless the attributable risk is high e.g. 1970 British Cohort Study (ongoing) http://www.cls.ioe.ac.uk/
Risk and Protection Is fish oil good for my mental health?
Ecological Studies Compare countries’ consumption of fish oil to their rates of depression Increased consumption of fish oil lowers a nation’s rate of depression (Hibbeln 2001), but eating fish is not the only difference among countries
Ecological Studies Large unit of analysis (e.g. countries) May identify population-level risk and protective factors Because the unit of observation is not the individual subject, they are subject to the ecological fallacy when they overlook important sources of variance
Risk and Protection Is running bad for my knees?
Case Control Identify a group of runners Then find a group of people who don’t run matched for age, sex, weight and other variables Test for associations between knee problems and being a runner
Case Control Inexpensive and practical Good for generating hypotheses Lacks a temporal dimension Unless data come from a population-based survey, cannot give incidence and prevalence data
Prognosis My husband has just taken 4 times the recommended dose of purple pills. What’s going to happen?
Case Report An otherwise healthy 60 year old man was prescribed 10 mg of vardenafil (Levitra, Bayer) for sexual dysfunction. Because this was ineffective, he increased the dose to 40 mg. Three hours later, he had a tonic- clonic seizure, seen by his relatives. On admission to hospital, neurological examination, brain magnetic resonance imaging, and electroencephalography after sleep deprivation were normal. Stress electrocardiography, echocardiography, and cardiac scan with dipyridamole test as well as carotid doppler ultrasonography did not show concomitant cardiac diseases. The man was told to stop using vardenafil. Two months later he had a new tonic-clonic seizure, four hours after taking 30 mg of vardenafil. At eight months' follow-up he is seizure- free without treatment. Pasquale Striano, Federico Zara, Carlo Minetti (professor of paediatrics), Salvatore Striano (2006). Epileptic seizures can follow high doses of oral vardenafil. BMJ;333:785.
Case Report Inexpensive and quick May draw attention to important clinical and research issues In rich detail, describes conditions and outcomes May not be representative, does not usually provide evidence of causation
Case Series Several case studies Draws attention to patterns in client populations Common in aetiological research
Effectiveness Does abstinence education reduce the likelihood of premarital sex?
Pre-post (single group) Take a class of kids Ask them if they will have sex before marriage They attend an abstinence-based education programme Ask them if they will have sex before marriage
Pre-post (single group) Inexpensive, generally easier than controlled studies Provides some evidence of temporal relationships Usually lacks a plausible counterfactual (i.e. what would have happened in the absence of intervention)
Between Group Look at all the kids in New York born in 1980 who were eligible for Head Start Compare those who attended to those who did not attend If possible, collect measures before and after attendance
Between Group Provides a counterfactual scenario, can give evidence of temporal relationships. Groups may differ on both measured and unmeasured variables, observed differences may be attributable to factors other than the intervention.
Effectiveness Does having regular contact with a social worker improve outcomes for fostered children?
Historical Control Compare children in foster care since the 1944 education act to children in foster care before then. If possible, include measures before and after enrolment for children in each group.
Historical Control Provides a counterfactual scenario Groups may differ on both measured and unmeasured variables, observed differences may be attributable to factors other than the intervention
Effectiveness Do intensive police crackdowns reduce gun violence?
Time-Series Identify areas with high levels of gun crime and identify peak times Repeatedly use crackdowns during periods of high crime Compare times with the intervention to periods without the intervention
Time-Series Provides a counterfactual scenario Times may be different Often requires complicated statistical analyses to control for differences in baseline variables, time trends, etc.
Conclusion What is your question? What types of study design might contribute to an answer? Think ‘Horses for Courses’