Coefficient of Determination (R2)

The coefficient of determination, denoted as R², is a statistic that measures how well a scatterplot's "best-fit" regression model represents the data.   The R² value is often used to judge the
“goodness of fit” of a trend line or curve through plotted data.

Students need to ensure that they interpret correlation coefficients correctly and refer to goodness of fit rather than assuming that the statistic infers the “reliability” or “accuracy” of their data. This is often a cause for misinterpretation and affects the data analysis criteria of the internal assessment.

The coefficient of determination ranges from 0 to 1, with values closer to 1 indicating stronger relationships between variables. A higher R2 value means a larger proportion of the variation in the dependent variable is explained by your model, indicating a better fit. For example, an R2 of 0.80 (or 80%) means that 80% of the variability in the dependent variable can be explained by its linear relationship with the independent variable. The remaining 20% of the variability is unaccounted for and is due to other factors or random chance. 

R² equal to 1  would be a perfect fit of all the data points to the "best fit" linear regression line.  A perfect R² is very rare due to the complexity of living organisms and multiple interacting variables.
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R² values between 0.6-0.9 often represent strong relationships in biological data. The linear regression model is a good fit for the data, meaning there is low scatter of data points around the regression line. 

R² values below 0.3 suggest weak associations that may not be biologically significant. The linear regression model explains very little of the data's variability. This means there is a large amount of scatter around the regression line, leading to greater uncertainty in any predictions or conclusions drawn from the relationship.

When to Use Correlation Coefficient (r) vs Coefficient of Determination (R²)

Use the correlation coefficient (r) to describe the direction and strength of a linear relationship between two variables. The correlation coefficient ranges from -1 to +1, making it ideal for identifying whether variables increase together (positive correlation) or move in opposite directions (negative correlation). Report r when the direction of the relationship is biologically meaningful and when conducting hypothesis testing for significance.

The coefficient of determination (R²) is used to quantify how much variation in one variable can be explained by another variable. R² is especially valuable when evaluating the effectiveness of a experimental design or the predictive power of a linear model. For example, if investigating factors affecting plant growth, an R² of 0.64 means that 64% of the variation in plant height can be explained by light intensity, while 36% is due to other factors.