Among the companies listed, gender diversity varies. A handful are at parity or better: Levo League, Hackbright Academy, and Yellowsmith—all companies, incidentally, with women at the helm—boast 67% women on their engineering teams. The Muse sits at 75% and Kabinet and Spitfire Athlete both hit 100%, though both have two-person engineering teams. On the other hand, 15 companies on the list are without a single female engineer: Treehouse, 37signals, and Causes.com among them.
While some of the smallest teams have 50% or more women, the numbers drop significantly once you look at engineering teams of 10 or more. When I broke down the data by size of engineering team, the averages looked like this:

My segmentation is somewhat arbitrary, but on average it looks like bigger teams have a lower percentage of women. It’s easier to get your percentages up when you’ve got a four-person technical team than it is when you’re hiring by the dozen.
Companies that participate in the counting do so as a signal to prospective employees that they are committed to diversifying their teams. A spokesperson at Mozilla—the largest company on the list, with a 500-person engineering team but only 43 women—told me that the project is “a reminder to keep pushing for more diversity.”
Until now, those of us writing about tech and gender have been making do with broad, US-centric data from the National Center for Women and Computing and the Anita Borg Institute, along with other sources we collect piecemeal. These data are tricky because they don’t typically differentiate between departments and roles within organizations: A woman in the HR department at Cisco will typically be counted as a “woman in computing,” whereas a woman software engineer at an investment company won’t. NCWIT suggests that women hold more than 25% of “computing occupations,” whereas my personal experience in the sector, which I’ve heard echoed by many colleagues, is that the numbers are significantly lower among software coders.
Even federal regulations have not provided us with reliable information: In March, when CNN was looking for data on women in tech, it was stonewalled by Silicon Valley giants whose size requires that they report diversity stats to the Department of Labor. While the government has that data, it won’t release it publicly, and most of the big companies aren’t talking. It’s unfortunate since data from these companies could be particularly valuable for benchmarking purposes given that their engineering teams are big enough to be statistically significant. When I tweeted earlier this year about that CNN story, one commenter suggested that sharing the data would result in a PR nightmare for the companies in question.
Marc Hedlund, VP Engineering at Stripe, has a different perspective: “If the first step is admitting you have a problem, I think in this context you have to say that very publicly for it to matter. Every company has a problem; your willingness to face it and work on it is what matters.”