When you have obtained results from your experiment or investigation, you then need to process them. In most investigations, you will have obtained a set of numbers as the results. One way to process these is to calculate the average value. There are three main types of average: median, mode and - the subject of this GCSE Biology quiz - mean.

It is always a good idea to repeat your experiment several times in order to get a larger amount of data. Once who have gathered enough data you can then calculate the **mean value**. The mean is one particular type of average. To calculate the mean you add up all the values and divide by the number of values in your sample (the sample size). When you work out the mean of a set of results, it is particularly useful as it '*smooths out*' the results and gives a more reliable overall answer.

Imagine that you have carried out an experiment looking at how temperature affects the growth of seedlings. Genetics can affect how quickly an individual plant will grow. The temperature might be slightly different for some seedlings too. The result would be that some grow more slowly than others. So to overcome these sources of error in your investigation, you should have planned to use a large number of seedlings and to measure the mean height. This ensures that no single seedling will influence the results so they will be more accurate and reliable. To get the most reliable results, you would need to use a large number of seedlings, the more you use, the better the mean will be.

The mean is one of the three most common types of **average**. The others are the median and the mode. The mode is the number that occurs most frequently in your data set and the median is the number that is at the centre of the list when it is written in order. If scientists talk about an average, the almost certainly are referring to the mean. So if you refer to the mean as the average in your exams or investigation, don't worry, you won't be penalised!