What is the product moment correlation coefficient?

• The product moment correlation coefficient (PMCC) is a way of giving a numerical value to?linear correlation?of bivariate data
• 

At AS Level you learned how to use linear regression models to describe a relationship between two variables. However, it is possible for two variables to have a relationship that does not fit a linear model, but still shows a pattern based on exponential growth or decay. A linear regression model is only appropriate if the PMCC is close to 1 or -1.

What forms can non – linear regression models take?

• If a?bivariate data set?appears to have a non – linear relationship it could fit an exponential model

How can non – linear regression models be used?

• Non – linear regression models can be used in much the same way as?linear regression models
• By coding the original data using logarithms (changing the variables) a regression line of?Y?on?X?can be found
  • This can be used to make predictions for data values that are within the range of the given data (interpolation)
  • Making a prediction outside of the range of the given data is called?extrapolation?and should not be done