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July 01, 2010

A Post About Nothing

Timothy McAdoo by Timothy McAdoo

Today I want to zero in on a special topic. This is not just an empty set of words: Let’s de-cipher another APA Style point!

The zero before a decimal point is known as a leading zero. Have you noticed that sometimes this zero is used in decimal values and sometimes it is not?

APA Style has a very simple guideline for leading zeros:

  • If a value has the potential to exceed 1.0, use the leading zero.
  • If a value can never exceed 1.0, do not use the leading zero.

Thus, because most units of measure have the potential to exceed 1.0, the leading zero is frequently needed. A value over 1.0 does not need to actually appear in the text. Here are just a few examples:

…was 0.75 in. tall by 0.95 in. wide.

Participants viewed, on average, less than 0.65 hr of the footage.

…had means of 1.01, 2.21, and 0.95, respectively.

…had 95% CIs [0.62, 1.12], [-2.44, 4.30], and [-3.19, -2.39], respectively.

There are some values that by definition can never exceed 1.0. The omission of the leading zero is a visual indicator of this restricted range. The most common cases are p values and correlations:

…was significant (p < .01).

…was significant (p = .001).

…was shown to be highly correlated (r = .71).

A consistent presentation of statistical values, both within a paper and across published articles, provides a visual symmetry that can help readers focus on content over form.

I hope this zippy post has helped nullify any confusion. If you’re still drawing a blank, you can also find this guideline and additional examples on page 113 of the Publication Manual of the American Psychological Association (6th ed.).


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