How “free” language affects clickthrough rate Experiment ID: #8727

Hoover Institution

Experiment Summary

Timeframe: 2/5/2018 - 3/27/2018

Hoover Institution leveraged a free eBook to grow the size of their email file. They were curious about the language used in the ads—the control ad had a call-to-action of “Get the facts on healthcare now”, but they knew from previous experiments that the word “free” was a powerful motivator. They set up a second ad with the call-to-action “Get the free eBook now” and launched an A/B test to determine a winner.

Research Question

Will adding the word “free” increase clickthrough rate?


C: "Facts" language
T1: "Free" language


Treatment Name Click Rate Relative Difference Confidence
C: "Facts" language 0.96%
T1: "Free" language 0.76% -20.7% 100.0%

This experiment has a required sample size of 16,665 in order to be valid. Since the experiment had a total sample size of 367,458, and the level of confidence is above 95% the experiment results are valid.

Flux Metrics Affected

The Flux Metrics analyze the three primary metrics that affect revenue (traffic, conversion rate, and average gift). This experiment produced the following results:

    20.7% decrease in traffic
× 0% increase in conversion rate
× 0% increase in average gift

Key Learnings

The new ads with “free” in the call-t0-action produced a 20.7% decrease in clickthrough rate. There are at least two possible reasons for this:

  1. The target audience is more interested in something with perceived authority and factual basis than something that is free, possibly due to the intellectual reputation of Hoover.
  2. The rest of the ad copy mentions “facts”, so the “free” language might produce a break in congruency rather than a lift due to incentive.

While a valid decrease was confirmed in clickthrough rate, there was no statistically valid decrease in conversion rate, which means that additional testing is needed on the ad and landing page.

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Experiment Documented by...

Jeff Giddens

Jeff is a Senior Vice President at NextAfter. If you have any questions about this experiment or would like additional details not discussed above, please feel free to contact them directly.