QQ Plot Explanation

Normality of Data

Straight Line Pattern: If the data points fall approximately along a straight line, it suggests that the data follows the theoretical distribution (e.g., normal distribution).

Deviations from Straight Line:

• S-shaped Curve: Indicates lighter tails than the normal distribution (platykurtic).
• Inverted S-shaped Curve: Indicates heavier tails than the normal distribution (leptokurtic).

Outliers

Points that deviate significantly from the straight line indicate outliers in the data. These points are extreme values that are far from what the theoretical distribution predicts.

Skewness

Positive Skew: If the data points curve upwards to the right, it suggests positive skewness (right-skewed).

Negative Skew: If the data points curve downwards to the left, it suggests negative skewness (left-skewed).

Tail Behavior

Heavy Tails: If the data points deviate upwards or downwards at the ends of the QQ plot, it indicates that the data has heavier tails than the theoretical distribution.

Light Tails: If the data points are closer to the line in the middle but deviate less at the ends, it suggests lighter tails.

Quantile Calculation

- Calculate the quantiles of the sample data.
- Calculate the corresponding quantiles of the theoretical distribution (e.g., normal distribution).

Plotting

- Plot the sample quantiles on the y-axis.
- Plot the theoretical quantiles on the x-axis.
- The 45-degree reference line helps to assess the alignment of data quantiles with the theoretical quantiles.

Perfect Fit: Data points lie on the reference line. The dataset follows the theoretical distribution.

Right-Skewed Data: Data points curve upwards to the right. The dataset has a long right tail.

Left-Skewed Data: Data points curve downwards to the left. The dataset has a long left tail.

Heavy-Tailed Data: Data points deviate upwards or downwards at the ends. The dataset has more extreme values than the theoretical distribution predicts.

Light-Tailed Data: Data points deviate less at the ends. The dataset has fewer extreme values than the theoretical distribution predicts.

Normality Testing: Checking if the dataset follows a normal distribution.

Model Assumptions: Verifying the assumptions of statistical models that assume normality.

Comparative Analysis: Comparing different datasets or distributions.

A QQ plot is a valuable tool for assessing how closely a dataset follows a theoretical distribution. It provides visual insight into the characteristics of the data, such as normality, skewness, kurtosis, and the presence of outliers. Understanding these aspects can help in making informed decisions about data analysis and statistical modeling.