**You will need**

- Copy of the graph above
- Ruler
- Pen

**Use a line of best fit to make predictions about hand reaction time**

- Place a ruler on the graph so it covers all the dots – make sure the ruler has the same downward slope as the data.
- Now, turn the ruler on its side (thin edge of the ruler on the page).
- Keeping the slope the same, move the ruler so that approximately half of the data points appear above the edge of the ruler and half are below it.
- Now, draw a line along the edge of the ruler. This is a line of best fit!
- Now, draw a cross on your line of best fit, directly above the data point for Year 6: label this P1.
- Draw another cross above the data point for Year 7: label it P2.
- Make a right angled triangle – rule a horizontal line (L1) from P1 until it lines up above P2.
- Now, rule a second line (L2) vertically to touch the line at P2.

**What’s happening?**

The length of L1 represents the change in year level. Read the year level from the horizontal axis. Since L1 runs from 6 to 7, it shows an increase of 1 year level.

The length of L2 represents the change in reaction time. Read the reaction time from the vertical axis. Because L2 goes from a higher value to a lower value, the reaction time decreases.

The value of L2 tells us how much the reaction time decreases every time a student’s year level increases by 1 year. When year level increases by 1 year, reaction time decreases by about 0.1 seconds. Since the line is straight, reaction time will reduce by the same amount (0.1 seconds) for each change in year level, no matter which two consecutive year levels we use on the line.

The graph tells you that the reaction time for Year 12 males is 0.03 seconds. Can you predict the reaction time for a boy who is one year older?

Take care when you make predictions. As with all statistics, there may be conditions outside your control that affect your data. Plus, decreases in time can’t be expected to continue forever – using your line of best fit, how quick would your reaction time be at 100 years of age?