Cross-secitonal studies

what are the two types of study

observational

intervention

descriptive study 3 features

objective not hypothesis

no association evaluation based on exposures and otucomes

form teh basis of hypotheses later - observations

what is a survey

estimate the frequency andd distribution of selected outocmes in a given population

how are analytical studies different to descriptive studies

comparison of 2 or more groups

researcher has no control over the group allocation

3 types of analytical observational study

cross-sectional

cohort

case-controol

retrospective analytical observational study

both exposure and outcome have occured at the time of study

2 features with OR for interventional studies

clinical trial or field trial

randomised or not - point is it is chosen

considerations of cross-sectional

background

study objective

population - inclusion and exclusion

sample size

outcome and exposure

analysis

(nice order)

characteristics of a cross-sectional study

studying 2 or more groups

measurment at a specific and defined point so no follow up

groups are determined by exposure and outcome

comparison is the prevalence of outcome in non-exposed and exposed animals

no follow-up

you can’t choose who is in each group

two types of prevalence and meaning

point prevalence - at one point

period prevalence - average or mid-interval over a set period

what are teh two things with sasmple size and precision and prevalence

as prevalence increases, sample size has to increase for all levels of precision to prove if true for whole population

as precision increases, population size has to icnrease to prove if true for the population to that degree

two methods of calculating OR

a/b/c/d

axb/cxd

what if the OR is lower than 1

the risk is greater of developing the outcome in teh non-exposed gropu than in the esposed group

limitations of cross-sectional studies

have to know the size of the population

prevalence is based on incidence and duration, so can’t distinguish between factors leading to more persistent infection, and more devleopment

over-represent cases with long duration

under-represent cases with a short duration

due to prevalence and therefore strength of association based on duration

disease with a high prevalence isn’t necessarily more importnat

current exposure status may not reflect status previously, other than with tombstone stuff