P Values and Comparing Groups

Why compare the HEALTH of groups?

Compare the health of 2* Groups? One time-point per pateint - CROSS-SECTIONAL STUDY

Cross-sectional study —→ One time point. One group surveyed to test associations between exposures and outcome/s

• Make 2 measurements per person: amount of cause vs amount of effect

• Measurements: predictor vs. outcome

Compare the health of 2* Groups? One time-point per pateint - ECOLOGICAL STUDY

Ecological study —→ community/population observed to test associations between exposures and outcome/s

Compare the health of 2* Groups? One time-point per pateint

Both studies focus on simultaneous observation of exposure and outcome, but difference is unit of observation

1) Cross-sectional study – focus on individual/patient level data

2) Ecological study – focus on population-level data (e.g. by country)

Compare the health of 2* groups? Two or more time-points per patient - COHORT STUDY

Cohort study —→ Prospective. 2+ time points.  Start disease-free participants followed up to see if they develop a disease (condition/outcome) of interest

• Usually with groups who differ at outset on some exposure/s of interest

Compare the health of 2* groups? Two or more time-points per patient - CASE-CONTROL STUDY

Compare the health of 2* groups? Two or more time-points per patient - RANDOMISED CONTROLLED TRIAL

Randomised controlled trial (RCT) —→ Prospective. 2+ time points. groups who are randomly allocated to receive intervention(s) versus comparator(s)

• Test safety and efficacy/effectiveness of interventions

• Example: Ronald Fisher’s fields to test efficacy of manures

Risk VS Odds: Smoking —→ Lung Cancer

What are the STRENGHTS of Case-Control Studies?

What are the WEAKNESSES of Case-Control Studies?

What is RANDOMISED CONTROLLED TRIAL?

Why and When are RCTs conducted?

What are the STRENGHTS of RCTs?

1) Minimise selection bias

2) Minimise measurement bias

3) Best single-study evidence for causal association between exposure (intervention) and outcome

What are the WEAKNESSES of RCTs?

1) Time-consuming

2) Difficult

3) Expensive

4) Researchers can over-rely on them

What is the GOAL of Statistical Testing?

What is a VARIABLE?

Variable = A parameter that you can or will measure/observe, which varies in your cohort

§Height, plasma glucose, eye colour, alive or dead (“living status”), treated or untreated

Variables - OUTCOME VARIABLE

 An outcome variable is the variable that supposedly results from other inputs.  It occurs later.

• What you think is “the effect that resulted from what you did”

• Also called the dependent variable

Variables - PREDICTOR VARIABLE

Variables - CONFOUNDING VARIABLE

Another (unwanted & unmeasured) variable or fact that is messing up your ability to correctly reach a conclusion

Variables - CONTINOUS VARIABLE

A continuous variable is a number that can go through many values and fractions: height, age

Variables - BINARY or DICHOTOMOUS VARIABLES

Variables - ORDINAL VARIABLES

Why are Statistical Tests used?

What is the CHI SQUARED TEST?

What is the difference between TYPE 1 and TYPE 2 ERROR?

Errors Bsed on Your Data: Type I VS Type II

What is the T-TEST?

What does T-Test COMPARE?

What is the P VALUE?

P Value: What You Want VS What You Get

Why are SMALL VALUES of P GOOD?

P Values: Example Comparing Two Randomly Generated Groups

What is the NULL HYPOTHESIS?

What is the EFFECT SIZE?

Effect Size: Cohen’s ‘d’

P Values VS Effect Size

What are CONFIDENCE INTERVALS (CI)

How do you compare HEALTH OUTCOMES?

Summary