6.2 Descriptive & Summary Statistics

Here are some general guidelines to follow when displaying descriptive or summary statistics:

  • Round numbers to one more digit for measures of centre (e.g. mean), and 2 more digits for measures of spread (e.g. sd) than was used in measuring the data
  • Units are preceded by a space within text passages:
    • e.g. "Average height was 34.2 cm (± 3.43 SEM)."

Describing Numerical Variables

  • Report mean with standard deviation, and additionally median with inter-quartile range for variables that exhibit a non-normal frequency distribution (e.g. is skewed) or that includes outliers
  • Parameter estimates (i.e. mean) should be accompanied by measures of uncertainty, i.e. the standard error of the mean SEM or confidence interval (notation: lower limit – upper limit); confidence limits: (lower limit, upper limit)
  • Confidence intervals are strongly encouraged because they inform about effect size
  • Measures of uncertainty for an estimate, such as SEM, can be preceded by a ± sign; do not make the common mistake of reporting a ± sign with a standard deviation, as it is not a measure of uncertainty in an estimate

Describing Categorical Variables

  • Report a frequency table (raw data) or a summary table with proportions for categories (the main descriptive statistic of interest), along with the confidence interval for the proportion if appropriate.