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

it might be feasible to test a new treatment on all known patients with arare condition, involving all known people with raised blood pressurein a study of a new anti-hypertensive drug would clearly beimpossible.Instead, a sample of people is usually selected and studied. This isperfectly acceptable – provided that a representative sample isselected from all of the people in the population of interest. Otherwise,the results obtained from the study cannot easily be generalised. Arepresentative sample:∑∑has (within reasonable limits) the same characteristics as thetarget clinical population – so accurately reflects the variations(both random and systematic) within that target populationcan usually be obtained by choosing a sufficiently large randomsample.A simple random sample:∑∑∑is selected in such a way that (in theory at least) every individualin the target population has an equal chance of being chosenis nearly always the most appropriate type of sample for a clinicaltrialis usually obtained in practice by taking all eligible individualswho present to the participating clinics during the period of thestudy (ie it is assumed that patients present themselves in arandom order).A stratified random sample:∑∑Basic concepts and definitionsmay sometimes be more appropriate for a medical survey orobservational studyis selected in such a way that different proportions of people arerecruited from particular sub-groups in the target population. ExampleA study is being planned to investigate how the lifestyle problemsencountered by people with a continent stoma vary with age. Theresearchers are most interested in the problems found by adolescents,but expect to find most stomas in older patients. If they are right, a3