Are there differences in automatic caption error rates due to pitch or speech rate?

So after my last blog post went up, a couple people wondered if the difference in classification error rates between men and women might be due to pitch, since men tend to have lower voices. I had no idea, so, being experimentally inclined, I decided to find out.

First, I found the longest list of words that I could from the accent tag. Pretty much every video I looked used a subset of these words.

Aunt, Roof, Route, Wash, Oil, Theater, Iron, Salmon, Caramel, Fire, Water, Sure, Data, Ruin, Crayon, New Orleans, Pecan, Marriage, Both, Again, Probably, Spitting Image, Alabama, Guarantee, Lawyer, Coupon, Mayonnaise, Ask, Potato, Three, Syrup, Cool Whip, Pajamas, Caught, Catch, Naturally, Car, Aluminium, Envelope, Arizonia, Waffle, Auto, Tomato, Figure, Eleven, Atlantic, Sandwich, Attitude, Officer, Avacodo, Saw, Bandana, Oregon, Twenty, Halloween, Quarter, Muslim, Florida, Wagon

Then I recorded myself reading them at a natural pace, with list intonation. In order to better match the speakers in the other Youtube videos, I didn’t go into the lab and break out the good microphones; I just grabbed my gaming headset and used that mic. Then, I used Praat (a free, open source software package for phonetics) to shift the pitch of the whole file up and down 60 Hertz in 20 Hertz intervals. That left me with seven total sound files: the original one, three files that were 20, 40 and 60 Hertz higher and finally three files that were 20, 40 and 60 Hertz lower. You can listen to all the files individually here.

The original recording had a mean of 192 Hz and a median of 183, which means that my voice is slightly lower pitched than average for an American English speakering women. For reference, Pepiot 2014 found a mean pitch of 210 Hz for female American English speakers. The same papers also lists a mean pitch of 119 Hz for male American English speakers. This means that my lowest pitch manipulation (mean of 132) is still higher than the average American English speaking male. I didn’t want to go too much lower with my pitch manipulations, though, because the sound files were starting to sound artifact-y and robotic.

Why did I do things this way?

  • Only using one recording. This lets me control 100% for demographic information. I’m the same person, with the same language background, saying the same words in the same way. If I’d picked a bunch of speakers with different pitches, they’d also have different language backgrounds and voices. Plus I’m not getting effects from using different microphones.
  • Manipulating pitch both up and down. This was for two reasons. First, it means that the original recording isn’t the end-point for the pitch continuum. Second, it means that we can pick apart whether accuracy is a function of pitch or just the file having been manipulated.


You can check out how well the auto-captions did yourself by checking out this video. Make sure to hit the CC button in the lower left-hand corner.

The first thing I noticed was that I had really, really good results with the auto captions. Waaayyyy better than any of the other videos I looked at. There were nine errors across 434 tokens, for a total error rate of only 2%, which I’d call pretty much at ceiling. There was maaayybe a slight effect of the pitch manipulation, with higher pitches having slightly higher error rates, as you can see:


BUT there’s also sort of a u-shaped curve, which suggests to me that the recognizer is doing worse with the files that have been messed with the most. (Although, weirdly, only the file that had had its pitched shifted up by 20 Hz had no errors.) I’m going to go ahead and say that I’m not convinced that pitch is a determining factor

So why were these captions so much better than the ones I looked at in my last post? It could just be that I was talking very slowly and clearly. To check that out, I looked at autocaptions for the most recent video posted by someone who’s fairly similar to me in terms of social and vocal characteristics: a white woman who speaks standardized American English with Southern features. Ideally I’d match for socioeconomic class, education and rural/urban background as well, but those are harder to get information about.

I chose Bunny Meyer, who posts videos as Grav3yardgirl. In this video her speech style is fast and conversational, as you can hear for yourself:

To make sure I had roughly the same amount of data as I had before, I checked the captions for the first 445 words, which was about two minutes worth of video (you can check my work here). There was an overall error rate of approximately 8%, if you count skipped words as errors.  Which, considering that recognizing words in fast/connected speech is generally more error-prone, is pretty good. It’s definitely better than in the videos I analyzed for my last post. It’s also a fairly small difference from my careful speech: definitely less than the 13% difference I found for gender.

So it looks like neither the speed of speech nor the pitch are strongly affecting recognition rate (at least for videos captioned recently). There are a couple other things that I think may be going on here that I’m going to keep poking at:

  • ASR has got better over time. It’s totally possible that more women just did the accent tag challenge earlier, and thus had higher error rates because the speech recognition system was older and less good. I’m going to go back and tag my dataset for date, though, and see if that shakes out some of the gender differences.
  • Being louder may be important, especially in less clear recordings. I used a head-mounted microphone in a quiet room to make my recordings, and I’m assuming that Bunny uses professional recording equipment. If you’re recording outside or with a device microphone, though, there going to be a lot more noise. If your voice is louder, and men’s voices tend to be, it should be easier to understand in noise. My intuition is that, since there are gender differences in how loud people talk, some of the error may be due to intensity differences in noisy recordings. Although an earlier study found no difference in speech recognition rates for men and women in airplane cockpits, which are very noisy, so who knows? Testing that out will have to wait for another day, though.

Google’s speech recognition has a gender bias

Edit, July 2020: Hello! This blog post has been cited quite a bit recently so I thought I’d update it with the more recent reserach. I’m no longer working actively on this topic, but in the last paper I wrote on it, in 2017, I found that when audio quality was controlled the gender effects disappeared. I take this to be evidence that differences in gender are due to differences in overall signal-to-noise ratio when recording in noisy environments rather than problems in the underlying ML models.

That said, bias against specific demographics categories in automatic speech recognition is a problem. In my 2017 study, I found that multiple commercial ASR systems had higher error rates for non-white speakers. More recent research has found the same effect: ASR systems make more errors for Black speakers than white speakers. In my professional opinion, the racial differences are both more important and difficult to solve.

The original, unedited blog post, continues below.


In my last post, I looked at how Google’s automatic speech recognition worked with different dialects. To get this data, I hand-checked annotations  more than 1500 words from fifty different accent tag videos .

Now, because I’m a sociolinguist and I know that it’s important to stratify your samples, I made sure I had an equal number of male and female speakers for each dialect. And when I compared performance on male and female talkers, I found something deeply disturbing: YouTube’s auto captions consistently performed better on male voices than female voice (t(47) = -2.7, p < 0.01.) . (You can see my data and analysis here.)

On average, for each female speaker less than half (47%) her words were captioned correctly. The average male speaker, on the other hand, was captioned correctly 60% of the time.

It’s not that there’s a consistent but small effect size, either, 13% is a pretty big effect. The Cohen’s d was 0.7 which means, in non-math-speak, that if you pick a random man and random woman from my sample, there’s an almost 70% chance the transcriptions will be more accurate for the man. That’s pretty striking.

What it is not, unfortunately, is shocking. There’s a long history of speech recognition technology performing better for men than women:

This is a real problem with real impacts on people’s lives. Sure, a few incorrect Youtube captions aren’t a matter of life and death. But some of these applications have a lot higher stakes. Take the medical dictation software study. The fact that men enjoy better performance than women with these technologies means that it’s harder for women to do their jobs. Even if it only takes a second to correct an error, those seconds add up over the days and weeks to a major time sink, time your male colleagues aren’t wasting messing with technology. And that’s not even touching on the safety implications of voice recognition in cars.

So where is this imbalance coming from? First, let me make one thing clear: the problem is not with how women talk. The suggestion that, for example, “women could be taught to speak louder, and direct their voices towards the microphone” is ridiculous. In fact, women use speech strategies that should make it easier for voice recognition technology to work on women’s voices.  Women tend to be more intelligible (for people without high-frequency hearing loss), and to talk slightly more slowly. In general, women also favor more standard forms and make less use of stigmatized variants. Women’s vowels, in particular, lend themselves to classification: women produce longer vowels which are more distinct from each other than men’s are. (Edit 7/28/2016: I have since found two papers by Sharon Goldwater, Dan Jurafsky and Christopher D. Manning where they found better performance for women than men–due to the above factors and different rates of filler words like “um” and “uh”.) One thing that may be making a difference is that women also tend not to be as loud, partly as a function of just being smaller, and cepstrals (the fancy math thing what’s under the hood of most automatic voice recognition) are sensitive to differences in intensity. This all doesn’t mean that women’s voices are more difficult; I’ve trained classifiers on speech data from women and they worked just fine, thank you very much. What it does mean is that women’s voices are different from men’s voices, though, so a system designed around men’s voices just won’t work as well for women’s.

Which leads right into where I think this bias is coming from: unbalanced training sets. Like car crash dummies, voice recognition systems were designed for (and largely by) men. Over two thirds of the authors in the  Association for Computational Linguistics Anthology Network are male, for example. Which is not to say that there aren’t truly excellent female researchers working in speech technology (Mari Ostendorf and Gina-Anne Levow here at the UW and Karen Livescu at TTI-Chicago spring immediately to mind) but they’re outnumbered. And that unbalance seems to extend to the training sets, the annotated speech that’s used to teach automatic speech recognition systems what things should sound like. Voxforge, for example, is a popular open source speech dataset that “suffers from major gender and per speaker duration imbalances.” I had to get that info from another paper, since Voxforge doesn’t have speaker demographics available on their website. And it’s not the only popular corpus that doesn’t include speaker demographics: neither does the AMI meeting corpus, nor the Numbers corpus.  And when I could find the numbers, they weren’t balanced for gender. TIMIT, which is the single most popular speech corpus in the Linguistic Data Consortium, is just over 69% male. I don’t know what speech database the Google speech recognizer is trained on, but based on the speech recognition rates by gender I’m willing to bet that it’s not balanced for gender either.

Why does this matter? It matters because there are systematic differences between men’s and women’s speech. (I’m not going to touch on the speech of other genders here, since that’s a very young research area. If you’re interested, the Journal of Language and Sexuality is a good jumping-off point.) And machine learning works by making computers really good at dealing with things they’ve already seen a lot of. If they get a lot of speech from men, they’ll be really good at identifying speech from men. If they don’t get a lot of speech from women, they won’t be that good at identifying speech from women. And it looks like that’s the case. Based on my data from fifty different speakers, Google’s speech recognition (which, if you remember, is probably the best-performing proprietary automatic speech recognition system on the market) just doesn’t work as well for women as it does for men.

Which accents does automatic speech recognition work best for?

If your primary dialect is something other than Standardized American English (that sort of from-the-US-but-not-anywhere-in-particular type of English you hear a lot of on the news) you may have noticed that speech recognition software doesn’t generally work very well for you. You can see the sort of thing I’m talking about in this clip:

This clip is a little old, though (2010). Surely voice recognition technology has improved since then, right? I mean, we’ve got more data and more computing power than ever. Surely somebody’s gotten around to making sure that the current generation of voice-recognition software deals equally well with different dialects of English. Especially given that those self-driving cars that everyone’s so excited about are probably going to use voice-based interfaces.

To check, I spent some time on Youtube looking at the accuracy automatic captions for videos of the accent tag challenge, which was developed by Bert Vaux. I picked Youtube automatic captions because they’re done with Google’s Automatic Speech Recognition technology–which is one of the most accurate commercial systems out there right now.

Data: I picked videos with accents from Maine (U.S), Georgia (U.S.), California (U.S), Scotland and New Zealand. I picked these locations because they’re pretty far from each other and also have pretty distinct regional accents.  All speakers from the U.S. were (by my best guess) white and all looked to be young-ish. I’m not great at judging age, but I’m pretty confident no one was above fifty or so.

What I did: For each location, I checked the accuracy of the automatic captions on the word-list part of the challenge for five male and five female speakers. So I have data for a total of 50 people across 5 dialect regions. For each word in the word list, I marked it as “correct” if the entire word was correctly captioned on the first try. Anything else was marked wrong. To be fair, the words in the accent tag challenge were specifically chosen because they have a lot of possible variation. On the other hand, they’re single words spoken in isolation, which is pretty much the best case scenario for automatic speech recognition, so I think it balances out.

Ok, now the part you’ve all been waiting for: the results. Which dialects fared better and which worse? Does dialect even matter? First the good news: based on my (admittedly pretty small) sample, the effect of dialect is so weak that you’d have to be really generous to call it reliable. A linear model that estimated number of correct classifications based on total number of words, speaker’s gender and speaker’s dialect area fared only slightly better (p = 0.08) than one that didn’t include dialect area. Which is great! No effect means dialect doesn’t matter, right?

Weellll, not really. Based on a power analysis, I really should have sampled forty people from each dialect, not ten. Unfortunately, while I love y’all and also the search for knowledge, I’m not going to hand-annotate two hundred Youtube videos for a side project. (If you’d like to add data, though, feel free to branch the dataset on Github here. Just make sure to check the URL for the video you’re looking at so we don’t double dip.)

So while I can’t confidently state there is an effect, based on the fact that I’m sort of starting to get one with only a quarter of the amount of data I should be using, I’m actually pretty sure there is one. No one’s enjoying stellar performance (there’s a reason that they tend to be called AutoCraptions in the Deaf community) but some dialect areas are doing better than others. Look at this chart of accuracy by dialect region:

Proportion of correctly recognized words by dialect area, color coded by country.

There’s variation, sure, but in general the recognizer seems to be working best on people from California (which just happens to be where Google is headquartered) and worst on Scottish English. The big surprise for me is how well the recognizer works on New Zealand English, especially compared to Scottish English. It’s not a function of country population (NZ = 4.4 million, Scotland = 5.2 million). My guess is that it might be due to sample bias in the training sets,  especially if, say, there was some 90’s TV shows in there; there’s a lot of captioned New Zealand English in Hercules, Xena and related spin-offs. There’s also a Google outreach team in New Zealand, but not Scotland, so that might be a factor as well.

So, unfortunately, it looks like the lift skit may still be current. ASR still works better for some dialects than others. And, keep in mind, these are all native English speakers! I didn’t look at non-native English speakers, but I’m willing to bet the system is also letting them down. Which is a shame. It’s a pity that how well voice recognition works for you is still dependent on where you’re from. Maybe in another six years I’ll be able to write a blog post says it isn’t.