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Four tabs illustrate 1) perfect positive correlation, 2) perfect negative correlation, 3) absence of correlation, and 4) intermediate level of positive correlation.

Tab Title: Positive Correlation

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r = +1 indicates a perfect positive correlation, i.e., as one measure goes up, so does the other.

Tab Title: Negative Correlation

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r = –1 indicates a perfect negative correlation, i.e., as one measure goes up, the other goes down.

Tab Title: No Correlation

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r = 0 means that there is no correlation between the two measures

Tab Title: Intermediate R values

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Intermediate "r" values provide a measure of how closely the observed data points conform to a straight line. Some authors say that the "r" value is a measure of the association between the risk factor and the disease, but this is incorrect. The slope of the line would be a measure of the strength of association.

The graph correlating meat consumption with the colon frequency of colon cancer demonstrates a positive correlation that would probably have an "r" value of about +0.8 or +0.9, meaning that the the data points were reasonably close to a straight line, suggesting a strong correlation between the two measures. If the correlation were +1 all of these plotted points would fall on a straight line.