QQkäyrät

QQkäyrät, also known as Q-Q plots or quantile-quantile plots, are graphical tools used in statistics to compare two probability distributions. They are particularly useful for assessing whether a dataset follows a specific theoretical distribution, such as the normal distribution.

The construction of a Q-Q plot involves sorting the observed data points and the theoretical quantiles of the distribution being tested. These sorted values are then plotted against each other. If the observed data follows the theoretical distribution, the points on the plot will lie approximately along a straight diagonal line. Deviations from this line indicate discrepancies between the observed data and the theoretical distribution.

For example, when testing for normality, the sorted data points are plotted against the corresponding quantiles of a standard normal distribution. If the plot shows a roughly linear pattern, it suggests that the data is approximately normally distributed. S-shaped curves might indicate skewness, while U-shaped curves could suggest heavy tails. Q-Q plots are a visual diagnostic tool, and their interpretation requires some understanding of statistical distributions. They are widely used in exploratory data analysis and in the assessment of model assumptions.