The wording of donation buttons makes a difference →

Cara Harshman explains a test the Romney campaign ran:

The team started optimizing for  donations with a test on the main call-to-action in the right upper corner of the homepage. They wanted to see whether button color – blue, green, yellow or red – and word choice – “Contribute,” “Support”, or “Donate” – impacted the likelihood of a visitor to click.

Overall they found that color did not have a definitive impact, but the word “Contribute” did show a statistically significant improvement of 10%.

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