Statistics for nMRCGP

Jo Kirkcaldy

Curriculum Condensed Knowledge

• Incidence and prevalence• Specificity and sensitivity• Positive and negative predictive values• Absolute risk and absolute risk reduction• Relative risk and relative risk reduction• NNT and NNH• Odds and odds ratio• Hazard ratios

Incidence and Prevalence

• Incidence– The number of new cases of a disease/condition

occuring in a population over a given time period

• Prevalence– The number of existing cases of a

disease/condition in a population at a specific point in time

Specificity and sensitivity

• Relevance is in screening and diagnostic tests

• Sensitivity = true positive rate– i.e. chance of a positive test result for people with

the disease

• Specificity = true negative rate– i.e. chance of a negative test result for people

without the disease.

Sensitivity and specificity

Disease No disease

Total

Test +ve a b a+b

Test -ve c d c+d

Total a+c b+d a+b+c+d

Sensitivity = a/a+c (x100)

Specificity = d/b+d (x100)

Positive and negative predictive values

• Measure screening performance (i.e post test probability)

• Positive predictive value– Chance of disease for person with positive test result

• Negative predictive value– Chance of no disease for person with negative test

result

PPV and NPV

• PPV = a/a+b• NPV = d/c+d

Disease No disease

Total

Test+ve a b a+b

Test -ve c d c+d

Total a+c b+d a+b+c+d

Risk

• Used in prospective studies

• Absolute risk – chance of developing disease/death over a set time period

• Relative risk - compares chance of developing disease/death in two groups e.g. with and without treatment over set time period

Deaths Survival Total

Control 32 67 99

New drug

21 79 100

Total 53 146 199

• Absolute risk of death for condition = 53/199 = 0.266 (x 100)

• Relative risk of death(risk ratio) = 21/100/32/99 = 0.21/0.323 = 0.65

• Relative risk reduction = 1-0.65 = 0.35 = 35%

NNT and NNH

• Related to risk

• Calculate how many patients need to save one life/harm one person

Deaths Survival Total

Control 32 67 99

New drug

21 79 100

Total 53 146 199

• NNT = 1/risk difference (=ARR)

• Risk difference = 32/99-21/100 = 0.323-0.21 = 0.113

• NNT = 1/0.113 = 9

• Need to treat nine people to save one life

Odds and odds ratioDeaths Survival Total

Control 32 67 99

New drug

21 79 100

Total 53 146 199

• Used in retrospective studies

• Odds of death– control = 32/99/67/99=

0.478– drug = 21/100/79/100=

0.266

• Odds ratio = 0.266/0.478 = 0.55

Hazard ratios• Reflects time survived to an invent i.e. survival

data.

• Complicated log calculation but essentially calculates ratio of rates at which an event happens.

• E.g if 1000 patients have an MI and 500 are thrombolysed hazard ratios could be used to describe the time to death in each group.

Curriculum Condensed Knowledge

• Incidence and prevalence• Specificity and sensitivity• Positive and negative predictive values• Absolute risk and absolute risk reduction• Relative risk and relative risk reduction• NNT and NNH• Odds and odds ratio• Hazard ratios

Further useful references

• ‘How to read a paper’ Trish Greenhalgh – pubilshed in serial form in BMJ or available as a book.