Paired data and correlation

A scatter diagram plots paired measurements: one point represents one item or person, with its two measurements on the axes. An upward trend shows positive correlation; a downward trend shows negative correlation. No clear linear trend suggests little or no linear correlation.

Here larger practice times tend to accompany higher scores, so the association is positive. The points need not lie on one line. A point far from the overall trend is an outlier and deserves investigation.

Correlation does not establish that one variable causes the other. For example, age or prior experience could affect both practice and score. Describe the relationship the data supports, not an untested explanation.