Nurs 236 - Week 7: Infectious Diseases Research: Study Designs & Measures of Association

Incidence Risk - comparing two groups

risk ratio = group A’s risk/group B’s risk

risk difference = groups A’s risk - Groups B’s risk

• group B is the reference (comparison) group

Incidence Rate - comparing two groups

Rate ratio = group A’s rate/group B’s rate

Rate difference = group A’s rate - Groups B’s rate

• Group B is the reference (comparison) group

Prevalence - comparing two groups

prevalence ratio = group A’s prevalence/group B’s prevalence

prevalence difference = group A’s prevalence - group B’s prevalence

• group B is the reference (comparison) group

Odds - comparing two groups

odds ratio = group A’s odds/group B’s odds

• group B is the reference (comparison) group

Study Design

plan for selecting participants and deciding how and when to collect information to answer a research question

• exposure: the factor being studied, including the treatment or intervention

• outcome: the health event or condition being measured

ex) the exposure is broad spectrum antibiotics, the outcome is C. diff; the exposure is influenza vaccination, the outcome is influenza infection; the exposure is injection drug use, the outcome is Hepatitis C infection

Experimental Study

has an intervention and can be random or non-random

• random is a randomized controlled trial

• non-random is a nonrandomized controlled trial

ex) researchers randomly assigns antibiotic A or B and compare recovery between groups

Observational Study

no intervention; the groups can be compared or not compared

• study that is used compared to examine a exposure-outcome association is called an analytic observational study

• study that is not used to compare is called a descriptive observational study

ex) clinicians choose antibiotic A or B during routine care; researches compare recovery between groups

descriptive observational study

describes frequency or patterns

• ex) case report or case series; descriptive cross-sectional study

Analytic Observational Study

depends on how the participants are selected and assessed; 3 types:

1. Cross-sectional study → exposure and outcome at one assessment time between two groups of different age (ex - young and old)'

   1. can be a past snapshot (exposure + outcome) or current snapshot (exposure + outcome); looks to see if the prevalence differ between exposure groups

2. Cohort Study → looks at exposure to outcome, group by exposure and assess later outcomes of two or more groups

3. Case-control study → outcome to exposure, groups selected by outcome, assess prior exposure

   1. compares past exposure by selecting cases and controls then looks at prior exposure'; looks to see if prior exposure differs between cases and controls

Descriptive Studies

main question → how common is the condition? who is affected, where, and when

purpose → describe disease patterns by person, place, and time

example → # of flu cases by age group

Analytic Studies

main question → is an exposure associated with an outcome?

purpose → examine associations

example → compare flu occurrence in vaccinated and unvaccinated groups

Prospective cohort

groups defined by exposure

• looks at exposure groups then follows forward and looks at new outcomes

• asks if incidence differs between exposure groups

Retrospective Cohort

groups defined by exposure

• looks at past records looking at exposure groups and later outcomes

• looks to see if incidence differs between exposure groups

Randomized controlled trial

intervention randomly assigned

• randomly assigns and intervention and follows forward to see the outcomes

• asks if outcomes differ between randomly assigned groups

Cross-Sectional Study

researches sample people from a defined population and assess exposure and current outcome during the same assessment period

• key limitation: researches usually cannot determine which came first: the exposure or the outcome

• exposure and current outcomes are assessed during ONE VISIT

• participants are NOT selected by exposure or outcome

• researches DO NOT FOLLOW participants for exposure or outcome

key takeaway: cross-sectional studies are a snapshot of exposure and current outcome

ex)

1. select a sample

2. assess at one point in time (ex. October 16, 2020); look at exposure (ex - indwelling urinary catheter present; yes or no) and look at outcome (ex - current UTI? - yes or no)

3. Describe or compare (point prevalence - proportion of patients with a current UTI) (UTI Prevalence - catheter present vs absent)

Point prevalence

the proportion with the outcome at a specific point in time

• common measure in cross-sectional studies

• ex) the percentage of surveyed U-M nursing students who have respiratory symptoms on survey day

Prevalence ratio and prevalence difference

compare prevalence between exposure groups

• ex) compare respiratory symptom prevalence between students reporting less frequent and more frequent hand hygiene

• common measure in cross-sectional studies

Cohort Study

researchers select and group participants by exposure without assigning the exposure; OBSERVATIONAL

• researchers compare risk or rate of developing outcomes over a defined period (can also be known as a longitudinal study); assesses whether exposed and unexposed participants develop the outcome

• key limitation: groups may differ in, underlying conditions, or other factors that may affect the results

• groups are defined by exposure

• participants are then followed to determine who develops the outcome

• the researcher foes not assign the exposure

• key takeaway: cohort studies compare subsequent outcomes between exposure groups

• useful for measuring incidence; studying rare exposures or multiple outcomes

prospective → occur after the study begins

retrospective → already occurred before the study begins

• both examine outcomes after exposure

Common measures used during a cohort study

incidence risk →

• denominator: people initially at risk

• comparison measure: risk ratio and risk difference

incidence rate →

• denominator: total person-time at risk

• comparison measure: rate ratio and rate difference

calculate each measure within each vaccination group, then compare the groups using a ratio or difference

Case-Control Study

researchers select the participants by outcome - cases have the outcome and controls do not when selected

• the controls come from the same population as cases

• researchers compare prior exposure between cases and controls; assesses to see whether cases and controls had the exposure before the outcome

• key limitation: poor control selection or inaccurate exposure information can distort the results

• key takeaway: select participants by outcome, then compare prior exposure

• useful for studying rare outcomes or several possible prior exposures

Exposure odds

number of previously exposed/number of unexposed

• calculated separately for cases and controls

• common measure for case-control studies

Odds ratio (OR)

exposure odds among cases/exposure odds among controls

• common measure of case-control studies

Why do we not use risk or rate for case-control studies?

because researchers select the participants by outcomes

• case-control samples do not directly provide population risk, incidence rate, or prevalence

Randomized Controlled Trait (RCT)

researchers randomly assign participants by chance (randomization) to an intervention group or a comparison group; EXPERIMENTAL

• researches follow both the groups and compare the outcomes over a defined period

• randomization helps balance out confounding factors

• key limitation: missing outcome data (e.g., loss to follow-up) can bias results

• has random assignment, comparison group, and outcomes assessed after intervention

common measures: incidence risk and incidence rate

• compare these two within each assigned group and then compare the groups using a ratio or difference

Two ways to compare groups (ratio and difference)

question

• ratio → how many times as high?

• difference → how much higher or lower?

comparison

• ratio → relative: divide the measures

• difference → absolute: subtract the measures

value when groups are equal

• ratio → 1

• difference → 0

example

• ratio → 2.0: twice as high; 0.5 - half as high

• difference → risk or prevalence of .1: 10 percentage points higher

units

• ratio → no units

• difference → % points when using percentages; cases per person-time for incidence rates

Calculating Prevalence

to get exposed groups ratio, you multiply the outcome times total (ex - 10×100 = .1); do the same with unexposed

prevalence ratio = exposed/unexposed

• when equal, value will be 1

• no units

prevalence difference = exposed - unexposed

• when equal, value will be 0

• percentage points

Calculating Incidence Risk

to get risk of exposed groups ratio, you multiply the outcome times total (ex - 10×100 = .1); do the same with risk of unexposed

risk ratio = risk in exposed/risk in unexposed

• when equal, value will be 1

• no units

risk difference = risk in exposed - risk in unexposed

• when equal, the value will be 0

• percentage points is the unit

Calculating Incidence Rates

to get risk of exposed incidence ratio, you multiply the outcome times total; do the same with risk of unexposed to get incidence ratio

• ex) 30 ppl exposed in 2,000 patient days = (30/2000) x 1,000 = 15 infections per 1,000 patient days

rate ratio → rate in exposed/rate in unexposed

• value when equal is 1

• no units

rate difference → exposed - unexposed

• value when equal is 0

• infections per unit of person-time

Odds Ratio

odds ratio = (exposure odds among cases)/(exposure odds among controls) = (a/b)/(c/d) = (axd)/(bxc)

interpretation

• OR > 1: odds of exposure are higher among cases than controls

• OR = 1: odds of exposure are equal among cases and controls

• OR < 1: odds of exposure are lower among cases than controls

interpret the results:

• the odds of ___ use before admission were __ times as high among patient who developed __ infection as among patient who did not

What does a clear interpretation identify

measure of association: risk, rate, prevalence, or odds

exposure (infection/treatment/intervention)

groups: who is being compared (unexposed/control)

outcome: what condition was measured

time: when or over what period

association

an observed statistical relationship between variables

correlation

type of association describing how two numerical variables vary together

causation

changing an exposure changes the likelihood of an outcome

association and correlation

they describe relationships but do not establish causation

confounding

another factor related to both exposure and outcome distorts their association

bias

systemic error in participating selection or measurement

chance

random variation in the findings

Research ethics

ethical review: need institutional review board (IRB) review before conducting experiments

informed choice: explain the study’s purpose, procedures, risks, and potential benefits. obtain informed consent when required; for minors, obtain parental/guardian and child assent as required

voluntary participation: respect he right to decline or withdrawal without penalty

privacy and confidentiality: protect participants and their personal information

safety: minimize risks, monitory safety as appropriate, and use ethically appropriate comparison group

fair selection: select participants fairly and provide additional protections when needed