The weather forecast for tomorrow suggests that there will be 20 mm of rain throughout the day. Caro says she can use the scatter diagram to predict how many umbrellas she would sell if the rainfall was 20 mm. Is she correct? Give a reason for your answer.

Caro is not correct. The data on the scatter diagram for daily rainfall ranges from approximately 2 mm to 12 mm. The forecast of 20 mm is well outside this range. Using a trend line to make a prediction for a value outside the range of the original data is called extrapolation. Extrapolation is unreliable because we do not know if the trend observed within the data range will continue beyond it. For example, after a certain point of heavy rain, everyone who needs an umbrella might have already bought one, so sales might level off or even decrease if people stay indoors.