Measuring incidence

1. The incidence of a disease is the number of new cases that appear in a population over a set time, usually one year.
2. Because populations differ in size, incidence is normally given as a rate, such as the number of new cases per $100000$ people per year.
3. The incidence rate is calculated using $$incidence\; rate=\frac{number\; of\; new\; cases}{population\; size}\times 100000$$
4. Using a rate allows a small town, a whole country and the whole world to be compared fairly.
5. Incidence data is collected from hospital and GP records and from large health surveys.
6. It is impossible to study everyone, so scientists collect data from a sample of people and use it to draw conclusions about the whole population.
7. For those conclusions to be valid, the sample must be representative, containing the same mix of ages, sexes, incomes and backgrounds as the population.
8. The sample must also be large, because results from a small sample are more likely to be affected by chance.
9. People should be chosen at random, so that the researcher cannot pick people who are likely to give a particular result.
10. Incidence in separate groups, such as different countries or age groups, is shown on a bar chart.
11. When a continuous variable such as alcohol intake is grouped into classes, the data is recorded in a frequency table and shown on a histogram.

Lifestyle factors

1. The four lifestyle factors with the greatest effect on the incidence of non-communicable diseases are smoking, diet, alcohol and exercise.
2. Smoking increases the incidence of lung cancer, several other cancers, bronchitis and cardiovascular disease.
3. A diet high in saturated fat, salt and sugar, or one that provides more energy than the body uses, increases the incidence of cardiovascular disease, obesity and type 2 diabetes.
4. A diet low in fruit, vegetables and fibre is linked to a higher incidence of bowel cancer.
5. Regularly drinking more alcohol than the recommended limit increases the incidence of cirrhosis, several cancers and high blood pressure.
6. In the UK, adults are advised to drink no more than $14$ units of alcohol a week.
7. Taking little exercise increases the incidence of cardiovascular disease, obesity and type 2 diabetes, while regular exercise reduces it.
8. Many people have more than one of these risk factors, so a change in one factor across a population can change the incidence of several diseases at once.

Local, national and global patterns

1. At a local level, incidence can differ greatly between neighbouring areas of the same town or city.
   1. Areas with more deprivation usually have higher rates of smoking and obesity, and so a higher incidence of lung cancer, heart disease and type 2 diabetes.
   2. Local differences also reflect how easy it is to buy healthy food, find safe places to exercise and see a doctor.
2. At a national level, incidence changes over time as the lifestyle of the whole population changes.
   1. In the UK, the proportion of adults who smoke has fallen steadily since the 1970s, and the incidence of lung cancer in men has fallen as a result.
   2. The fall in lung cancer came many years after the fall in smoking, because cancers take a long time to develop.
   3. Over the same period, the proportion of adults who are obese has risen, and the number of people with type 2 diabetes has increased sharply.
   4. National laws, such as banning smoking in enclosed public places, can be followed by falls in the incidence of smoking-related diseases.
3. At a global level, non-communicable diseases cause around three quarters of all deaths.
   1. Incidence has been highest in high-income countries, where diets are rich in fat and sugar and many jobs involve little physical activity.
   2. In many low- and middle-income countries, incidence is now rising quickly as diets change, car use increases and smoking becomes more common.
   3. Comparing countries helps scientists to identify which lifestyle factors matter most, because countries differ in their diet, smoking and drinking habits.

Correlation and cause

1. Data on a lifestyle factor and a disease is often plotted on a scatter diagram, with the lifestyle factor on the $x$-axis and the incidence of the disease on the $y$-axis.
2. If the points slope upwards, there is a positive correlation, meaning the disease is more common where the factor is higher.
3. If the points slope downwards, there is a negative correlation, such as the incidence of heart disease being lower in groups who take more exercise.
4. If the points show no pattern, there is no correlation.
5. A correlation on its own does not prove that the factor causes the disease.
   1. The link may be due to chance, especially when the sample is small.
   2. A third factor may affect both variables, for example older people may exercise less and also be more likely to have heart disease simply because of their age.
6. Scientists accept a causal link when several kinds of evidence agree.
   1. The same correlation is found repeatedly in large, representative samples from different countries.
   2. There is a plausible mechanism that explains how the factor affects the body, such as chemicals in tobacco smoke damaging the DNA of lung cells.
   3. The risk increases as exposure increases, so heavy smokers have a greater risk than light smokers.
   4. Reducing the factor reduces the incidence, as shown by the falling risk of lung cancer in people who stop smoking.