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Random Sample - Every person should have an equal chance of being chosen for your sample to make it fair and avoid bias.  A quick way of doing this is to give each student a random number and then sort the data on this number which produces a random list of the data.

### 1. Type =RAND() in the first free cell to the right of the first line of data and press Enter to insert a random number.

2.

Click on this cell again and move the cursor to the bottom right of the cell until it changes to a black cross.  Drag down until you reach the bottom of the data.

3.

To mix up the data, highlight the cell to the right of the first random number.  Select the Data menu and Sort.  Sort by column 5 and this will mix up all the data.

4.

Now select Data menu and Sort and Sort by Year Group Then by Gender.

5.

Select the number of calculated students from each group and copy to a separate sheet

Calculations and evidence required for your pilot study:

One scatter graph showing all sample for pilot study

Correlation coefficient for all heights and weights in your sample

Scatter Graphs – Are used to compare the relationship (correlation) between two types of data.

Correlation Coefficient – This is a more accurate method to compare correlation.  It uses the mean of each set of data and looks at the distance away from the mean of each point.  The formula, which is known as the Product Moment Correlation Coefficient or r is

(where and are the means of the x and y values respectively)

The value of r determines correlation.  It is always between –1 and 1.

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