Working with Counts

Question 1

Interpreting the output of a chi-square test

Answer 1

• the test statistic gives an ‘overall’ result
• we can break this result down using standardised residuals
• standardised residuals have a direct relationship with the test statistic (they are a standardised version of the difference between observed and expected frequencies)
• these standardised are z-scores (e.g. if the value lies outside of the range between –1.96 and +1.96 then it is significant at p < 0.05)

Question 2

Risk ratio

Answer 2

(a/(a+b))/(c/(c+d))

Question 3

Odds ratio

Answer 3

(a/b)/(c/d)
= 1 => the event is equally likely in both groups
> 1 => the event is more likely in the first group
< 1 => the event is less likely in the first group

Question 4

Fisher’s exact test

Answer 4

• a statistical significance test used in the analysis of contingency tables
• use Fisher’s exact test when the total sample size is less than 1000