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The Rule of Thumb

·3 mins

“Here’s a good rule of thumb”.

We’ve all said it. Especially those of us who work in data. We’re often asked to generalize the results of findings, or to look for meaningful signal in noise. The expression “rule of thumb” is a common way for analysts to describe what we’re seeing in data, and allow others to follow the narrative.

While the exact origins of the term are contentious, there is enough evidence to suggest that the history of the expression is steeped in misogyny. Folk etymology from Victorian England alleges an English law where a man was permitted to beat his spouse provided that the “implement used was a rod or stick no thicker than a man’s thumb’. Whether or not this was actual law is debatable, but the rumour of the law itself was used to justify wife-beating in the centuries that followed. In a cruel twist of fate, the rule of thumb, sadly enough - became a rule onto itself.

In the 18th century, several court rulings in the United State referred to a supposed common-law doctrine which the judges believed “had once allowed wife-beating with an implement smaller than a thumb”. From Wikipedia:

“None of these courts referred to such a doctrine as a rule of thumb or endorsed such a rule, but all permitted some degree of wife-beating so long as it did not result in serious injury”

This rule was used to justify and defend domestic violence through the 18th and 19th centuries:

  • An 1824 court ruling in Mississippi stated that a man was entitled to enforce “domestic discipline” by striking his wife with a whip or stick no wider than the judge’s thumb.

  • In a later case in North Carolina (State v. Rhodes, 1868), the defendant was found to have struck his wife “with a switch about the size of this fingers”; the judge found the man not guilty due to the switch being smaller than a thumb

  • Through the late 19th century, while most states had outlawed wife-beating, there was a common belief in parts of the United States that a man was permitted to beat his wife with a stick no wider than his thumb.

There has been debate in the past about the origin of the expression, and whether it was ever an actual law in Victorian England. Some etymologists prefer that the origin is linked to the cloth trade - the width of the thumb, or “thumb’s breadth” was used as the equivalent of an inch in the cloth trade. But it is impossible to deny that the rule was used to justify domestic violence for centuries, and while has become a part of our vernacular and we’ve appropriated it’s use - the negative connotation of the term is enough for me to stop using it in my work. The expressions: “a good approximation” or “a general rule” are appropriate alternatives to “a rule of thumb”.

At a time of heightened sensitvity to injustice around the world, this is the least that I can do.

Further reading here: Rule of Thumb and the Folklaw of the Husband’s Stick

Saadat Qadri
Saadat Qadri
I’ve been fortunate to work with and learn from data, product and growth leaders at companies such as InVision, CB Insights, Breather, Karbon, FreshBooks, Wealthsimple, among others. I love getting my hands dirty in helping build infrastructure to power insights and growth. I’ve helped build data teams from scratch, and I’m learning how to a better manager every day.