Biodiversity is a measure of how many different species live in an ecosystem. Sampling occurs when scientists look at part of a population to draw conclusions about the whole.

There are three different types of average which are all useful in science. They are called the mean, median and mode.

The mean is the most common type of average we use. To calculate the mean you add all the values together and divide by the total number of values.

Two students completed an ecological investigation into the dandelions on the school field. They randomly placed ten quadrats in shaded and sunny areas, in order to count the dandelions in each. Their results are below.

Quadrat | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
---|---|---|---|---|---|---|---|---|---|---|

Number of dandelions per quadrat in shade | 4 | 4 | 6 | 0 | 1 | 4 | 2 | 3 | 6 | 5 |

Number of dandelions per quadrat in sunlight | 6 | 5 | 7 | 8 | 4 | 5 | 8 | 5 | 5 | 3 |

The students wanted to compare their results, thus they calculated the mean for each. They added up all the dandelions in the shade, which came to 36 and all those in the sun which came to 57. They divided each of these numbers by ten to calculate the two means, as there were ten numbers from the ten quadrats.

Quadrat | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | Total | Average |
---|---|---|---|---|---|---|---|---|---|---|---|---|

Number of dandelions per quadrat in shade | 4 | 4 | 6 | 0 | 1 | 4 | 2 | 3 | 6 | 5 | 35 | 3.5 |

Number of dandelions per quadrat in sunlight | 6 | 5 | 7 | 8 | 4 | 5 | 8 | 5 | 5 | 3 | 57 | 5.7 |

The number of dandelions per quadrat is given to one significant figure. Usually the same number of significant figures would be used in the results of a calculation, but in this case when finding the mean of a series of integers, giving the answer to two significant figures is acceptable. Using three significant figures (eg 3.50) would be unacceptable in this case because it implies an inappropriate level of precision that did not exist in the original measurement of data.

To calculate the median, a set of numbers are placed in increasing order of size. The median is the middle number in the list. The two students took an even number of readings and they calculated the median as the mean of the two middle numbers.

The median for shade is 4 because both middle numbers are 4 and the median for sun is 5.5, as it is halfway between 5 and 6.

Number of dandelions per quadrat in shade | 0 | 1 | 2 | 3 | 4 | 4 | 4 | 5 | 6 | 6 |
---|---|---|---|---|---|---|---|---|---|---|

Number of dandelions per quadrat in sunlight | 3 | 4 | 5 | 5 | 5 | 6 | 6 | 7 | 8 | 8 |

The mode is the value that appears the most often. In the shade, the mode is four because there are three values of four. In the sun, it is five because there are three values of five.