essential statistics for medical examinations(final).pdf - PasTest

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1 Basic concepts and definitions1.1 Why are statistics necessary?THE RESULTS OBTAINED FROM CLINICAL TRIALS/SURVEYS ARE OFTEN WRONG!Data obtained from studies involving human beings (or, indeed, anybiological system) vary, often considerably. There are many reasonsfor this, including:∑∑∑∑Studies are usually done on samples – these rarely (if ever) haveexactly the same characteristics as the population they are takenfrom.Observations are rarely exact – some error is usually involvedwhen a measurement is taken.Biological systems often fluctuate naturally, even when theyappear to be in a steady state.There may be external influences (both random and systematic)acting on the observations.A simple listing of the data obtained in a clinical study is unlikely toshow any obvious patterns – the variation will conceal the real results.In addition, the chances are that the variation will also make thefindings of the study inaccurate and imprecise – so drawing correctand sensible conclusions may prove to be difficult.The effects of measurement error/variation cannot be removedcompletely, but good practice can help to reduce them. A good studydesign will help to limit and control the known sources of variation ina set of observations. However, there may also be some unknownsources of variation. By definition, these cannot be controlled oraccounted for. Consequently, there will be uncertainty in the1
Basic concepts and definitionsconclusions drawn from even the best conducted experiment.In a sense, Donald Rumsfeld (US Defence Secretary) got it right in2003 when he made his now (in)famous statement during the IraqWar:as we know, there are known knowns – there are things we know weknow. We also know there are known unknowns – that is to say we knowthere are some things we do not know. But there are also unknownunknowns – that is, there are things we do not know that we do not know.A good statistical analysis will estimate the amount of variation due toeach known source of error and will extract the information needed tointerpret the results of the study. More importantly, it will indicate thelevel of confidence (certainty) that can be attached to these results.STATISTICS IS:THE SCIENCE OF MEASURING THE (KNOWN) SOURCES OFVARIATION PRESENT IN AN EXPERIMENT.THE ART OF DRAWING CORRECT CONCLUSIONS FROMINACCURATE INFORMATION.1.2 Populations and samplesTarget population: all people with the disease/condition ofinterest to whom the conclusions of the study will be applied.Study population: the people actually available and accessible forstudy – this is usually limited to the subgroup of people with thedisease/condition of interest attending the clinics where the studyis being conducted.Ideally, a clinical trial or survey should be conducted on every personwith the disease or condition of interest. This is rarely possible. While2
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