Incidence & Prevalence

Imagine that a while ago you were celebrating a party with friends. You look back at a group photo and notice that some of your friends have their hair coloured red. There were 100 guests and 30 of them had red hair – the prevalence of red hair at the party was 30%.

If we were talking about a health event, we would ask: ‘What is the prevalence of that health event?’, but the mathematical calculation would be exactly the same. We could express the prevalence as percentages (here 30%) or alternatively as cases per 1,000 persons (here 300 per 1,000).