While performing the analyses above, we looked at the impact of capitalization on open rates. The results were quite surprising. First of all, the use of an entirely capitalized subject line resulted in slightly higher open rates than usual for a given user/list.
We then cast a wider net to see what the impact of having at least one fully capitalized word was, and the effect was slightly negative. Both of these impacts, though small, are statistically significant. It’s worth noting that our methodology compares campaign performance to other campaigns from the same user to the same list. That means completely capitalized subjects result in higher open rates for senders that also use mixed-case subject lines.