Home PC News How Duolingo uses AI in every part of its app

How Duolingo uses AI in every part of its app

Language studying has surged in the course of the pandemic. Duolingo, which is synonymous with gamified language studying, noticed its quickest development interval this March with a 101% international enhance in new customers. From those that merely have extra time on their fingers to college students making an attempt to maintain up within the pandemic school year, the app is a large boon. All that further information isn’t going to waste — as a result of Duolingo invested in AI early, the app retains getting higher because it grows past its 30 million month-to-month lively customers (as of December 2019).

“One of the things people don’t know is that even though the Duolingo is very gamified and it just looks very cutesy, we actually record everything you do to try to basically have a model of what you know,” Duolingo CEO Luis von Ahn instructed VentureBeat. We spoke to von Ahn about all of the methods Duolingo makes use of AI after which adopted up with the corporate’s analysis director, Burr Settles, who joined in 2013 (Duolingo was based in 2012). “We hired this guy named Burr who has a PhD in AI,” von Ahn stated when describing the corporate’s first foray into AI. “He came in and the idea was ‘try to figure out how to use AI to improve Duolingo’.”

We’ve already performed deep dives into how Duolingo makes use of AI to humanize digital language classes and to drive its English proficiency exams. This is a more in-depth have a look at all facets of the app itself, together with the AI behind Stories, Smart Tips, podcasts, studies, and even notifications.

Duolingo AI

All of that provides as much as a superior language studying expertise, Duolingo says. Indeed, the corporate at the moment published a report claiming its customers carried out as properly on studying and listening exams as college students who took 4 semesters of college courses, in half as many hours.

Spaced repetition

As you employ it, Duolingo builds an exceptionally detailed profile primarily based on what you realize and what you don’t know.

“We know everything by individual word,” von Ahn stated. “We have a whole space repetition system. We know how many times you’ve seen that word and we have an idea of how long it will take you to forget this word.”

Duolingo screenshots for an English-speaking student learning French

The spaced repetition system was the very first AI undertaking the corporate undertook, again in 2013. The mannequin is ready to predict if you’ve forgotten one thing since you haven’t seen it very ceaselessly, or very not too long ago. To at the present time, Duolingo makes use of it to assist choose which challenges it can put right into a observe session for you.

“That’s still in production,” Settles stated. “That’s a project that we actually hadn’t really touched for about seven years and over the last quarter we’re actually reviving that and finally going back and improving on those with some things we’ve learned. And also in 2013 we built a computer adaptive placement test. When you first sign up for a course, you can take about five minutes and it will place you into where you belong in the course. And we’re also doing some active improvements on that. That second project was the inspiration for the Duolingo English tests. VentureBeat recently did a rather in depth thing on that, but that’s AI end to end.”

Duolingo words that need practice

Within every lesson, Duolingo decides which workouts to offer you primarily based on the phrases and ideas the app believes you should observe. The particular workouts you’re served fluctuate, so every general lesson of workouts finally ends up being totally different for everybody.

“We may have a list of 20 words that we’re trying to teach you. That is the same for everyone,” von Ahn defined. “But those words we have some latitude about how we teach them to you. For example, we may teach you the word ‘chair’ by giving you the sentence, ‘I love this chair’ or we may give you the sentence ‘I sat on this chair.’ And we make the choice about which one to teach you the word chair with based on what we think may be better for you.’”

If you’re battling previous tense and Duolingo has an array of workouts in numerous tenses, it can choose the previous tense ones inside the lesson you’re doing, simply to just be sure you observe that extra.


This is all attainable because of a machine studying implementation affectionately referred to as Birdbrain.

“Generally for every exercise we have a really good idea of how hard that exercise is for you,” von Ahn stated. “For every sentence, before we give it to you we have a probability of what likelihood there is that you’re going to get that sentence right, that exercise right. It gives us no explanation about what you know, and what you don’t know, it just says look ‘Emil, for the sentence the man is on the chair, has a 93% chance of getting it right.’”

Furthermore, Birdbrain adjusts the problem inside a lesson primarily based on particular person issue of a sentence, for you particularly. “And we use that to calibrate difficulty,” von Ahn stated. “If you’re getting everything right we say ‘Let’s give you something that that we think you only have a 70% chance of getting right to see whether you get it right or not.’ If you’re getting a lot of things wrong we actually start giving you things that are easier.”

Duolingo right answer

Think of Birdbrain as the final word personalization studying system.

“It’s a massive system that trains every night using the half a billion or so lessons from the day before,” Settles defined. “As a byproduct of making these predictions, it models how hard the challenges are, as well as how proficient the users are. And so we’ve got this micro service now within the system where, what we call session generator that’s the system that constructs your lesson just for you when you go in to do a lesson or a practice session. And it would say ‘okay, here’s like 200 challenges that I could put into this. I’m only going to use 14 of them. But here 200 or so that might fit.’ Birdbrain will come back and say well out of those 200, here’s the probability threat for each one of those. And then session generator can use that to help pick which challenges will be in there. It can use it to sequence or order which challenges will be in that particular lesson.”

Duolingo can begin supplying you with customized AI-generated classes or strategies inside classes after you’ve accomplished a couple of hundred workouts, or simply 5 or 6 classes. The system is pretty new — Duolingo solely began creating it in October 2019 and launched a product characteristic that used it in March.

“Multiple teams use this service to fine tune the experiences that they own,” Settles stated. “And so over time, the fraction of sessions that are being personalized with Birdbrain keeps going up.”

Last month, 6%-8% of Duolingo periods had been being personalised by Birdbrain. Today the quantity is at 12% as groups on the firm maintain discovering new methods to make use of it.


For Birdbrain’s personalization to be really helpful, Duolingo must know why you’re failing sure workouts.

“When you get a challenge right or wrong, at this point Birdbrain doesn’t actually know why you got it right or wrong,” Settles stated. “If there was a misconjugation or if it was a noun adjective agreement, or if you just typed in word salad, it doesn’t distinguish between those as far as Birdbrain is concerned.”

Duolingo makes use of the whole lot it is aware of about each train, which is tagged with as a lot element as attainable (a part of speech, sentence construction, tense, and so forth), so it could possibly determine what accountable. Those tags had been as soon as performed manually, however not anymore.

“We do a lot of it automatically now of the tagging for every exercise,” von Ahn stated. “And then, whenever you get it wrong, we have this algorithm that’s called Blame that we try to assign blame of why you got it wrong. So when you enter and you get it wrong, we try to figure out like ‘Oh it’s because you didn’t know the word for that or it’s because you knew the word for that but you don’t know how to make it go into the past tense.’ And then, we have a pretty good idea of the things that you often get wrong.”

Duolingo wrong answer

There isn’t any separate algorithm if you get the train proper, however Duolingo does monitor that as properly.

“If it’s right, we give you credit. We say ‘okay, he just did an exercise that has these words and these concepts and got it right therefore our confidence that this person knows these concepts went up. But if you get it wrong, it’s much harder because Blame is trying to figure out which of the concepts is the culprit of why you got it wrong. And sometimes we can’t because you enter an answer that is so off that who knows. But most of the mistakes that people enter, usually it’s like one or two things off. And we try to figure out what concept that you didn’t know. Did you not know the word for it? Did you not know the gender of the word for it? Did you not know how to conjugate into the past tense? Did you not know that adjectives come before the noun?”

Blame can spit out a number of causes for why you bought one thing flawed. And after all, the extra errors you make, the more durable it’s to decipher. “At some point it just kind of gives up,” von Ahn stated.

If you realize you’re going to get a problem flawed, however you acknowledge a phrase, translating simply that phrase can be higher than simply responding with gobbledygook. “It would definitely be better for our model. Our model would have a better opinion of you.”

Conversely, if you happen to get the entire thing proper, Duolingo doesn’t essentially assume you realize all the ideas therein — perhaps you simply guessed appropriately. “That’s exactly right,” von Ahn confirmed. “This is all probabilistic. Now we have now a bit extra confidence that you realize the phrase for ‘banana.’

Active studying

Knowing learn how to translate particular person phrases isn’t sufficient for efficient communication in a brand new language. Sentence building and understanding is simply as vital. Last yr, the corporate began engaged on a characteristic referred to as Smart Tips. For some errors that you simply make, Duolingo tries to determine the foundation trigger so it could possibly give you a well timed tip. For instance, if Duolingo notices that you simply entered the precise phrases however within the incorrect order, it may give you a corrective grammar tip proper after it spits out that your enter was incorrect.

Duolingo Smart Tips

Seems easy sufficient, proper? It seems Smart Tips isn’t simply easy machine studying.

“That required some major creativity,” Settles stated. “Each challenge and each response gets run through what is a pretty textbook natural language processing pipeline. Here’s the sentence, these are all the nouns. This noun is masculine, it’s plural, and it is the subject of this verb. All of that stuff is pretty textbook. But then, figuring out that this person made this specific mistake — they got the word order wrong or they got the gender of the noun and adjective agreement wrong. Those are rules that are human crafted on top of the textbook, natural language processing pipeline.”

Settles’ PhD is in active learning and he wrote a guide about machine studying algorithms that ask questions. Rather than simply passively consuming information and studying to foretell one thing, they develop a speculation or a number of hypotheses and take a look at to determine which is the precise one by asking questions of a human oracle.

“What we’re doing here is we run an NLP for the correct answer and we run the NLP pipeline on the wrong answer,” Settles stated. “We look at the difference between those and try to come up with a bunch of explanations of what’s wrong. We know it’s wrong. But what’s wrong about it? And then do that in aggregate over a couple million exercises every day. And then make suggestions to a human of like, ‘Hey, this is what I think is going wrong in a lot of these challenges.’ And then it will propose some rules, and they can kind of click on the rules and see examples of the correct answer and the incorrect answer that would be covered by that rule. They kind of collaborate with the AI to come up with the right set of rules.”

It’s this forwards and backwards between the AI and the human workers that ends in guidelines for widespread grammatical error patterns. The course of requires aggregating all the information concerning the errors that Duolingo customers make on daily basis. Duolingo’s workers then decides what’s a rule and whether or not it needs to be revealed as a tip. Some compiling and optimization comply with to make sure that the brand new tip reveals up shortly in your telephone if you make the corresponding mistake. And then it occurs another time, with new kinds of errors and guidelines revealed.


Duolingo even makes use of AI to enhance the effectiveness of its notifications. The app sends you a notification every day to remind you to observe your, for instance, Spanish.

“We use AI to figure out when to send them to you and what to tell you,” von Ahn stated. “We trained the whole system trying to figure out when is the best time to send the notification based on your own activity. We know your activity on Duolingo and then for a given day we’ve watched all the days in the past when you’ve used Duolingo, and then we pick a time when to best send you the reminder and also what to say in that reminder. We’ve made pretty big gains in terms of number of people coming back.”

Duolingo notifications

After Duolingo applied its novel bandit algorithm, the corporate noticed a 2% enhance in new consumer retention sooner or later by way of one week after they downloaded the app.

That won’t appear to be rather a lot, but it surely’s a major enhance if you happen to think about that the one enter information is when the app is used. After only a few days, Duolingo can optimize when to ship you the notification. Even sooner or later of information is beneficial.

“It’s pretty good actually,” von Ahn. “It’s interesting. If we only have one day of information about you, you know what the system does? It sends you the notification at exactly the same time the next day. Turns out that’s actually pretty good. After we have a few days we get better and better. Probably after about a week of usage, we get a pretty good idea of when you use Duolingo. Sometimes it may vary by day of the week, so we have noticed that for some people it does something a little different for the weekends than during the weekdays. The system is kind of all trained using just data from you, but it gets pretty good, pretty fast.”

Unlike most AI implementations the place there may be all the time a ton of potential for enchancment, this appears like a solved drawback. “I don’t know if it’s a solved problem but we feel pretty good with what we have there and it’s hard to imagine that we can do a lot better,” von Ahn stated. “Like perhaps we are able to perform a little higher, but it surely does a reasonably good job.

Logistic regression

Whenever you submit a solution to a problem, and Duolingo says you bought it flawed, you will have the choice to hit the Report button. If you assume you bought it proper, you possibly can attraction.

Duolingo reports from English users learning Chinese

“We get about, somewhere between half a million and a million of those every week, and 90% of them are junk,” Settles stated. “They’re either accidental taps or the people are wrong but they think they’re right. But about 10% of those are bugs in the course. Or not necessarily bugs, but things that are acceptable. Maybe they’re not the most fluent or idiomatic way of doing it, but they’re correct, and so we should modify the course content to include those. But it’s a real needle in the haystack kind of process for the course content maintainers and developers.”

To deal with this problem, the staff constructed a machine studying system utilizing a logistic regression algorithm that may floor the helpful studies.

“For a while we just sort of ranked the reports by how many people submitted this particular exact sort of report,” Settles stated. “And that helped a little bit. But in the process of doing that we collected a lot of training data; well this is actually correct and this is not correct. So we were able to train a machine learning model to predict which reports are likely to be accepted by our contributors. And we did this in a vastly kind of multilingual way so that now there’s an interface that basically rank orders all of the reports so that they can find the most salient ones to fix first.”

Duolingo report prioritization

It’s vital that Duolingo is rating the reports and never simply discarding the much less helpful ones — in any case no algorithm is ideal. Plus, there are nonetheless too many studies for the staff to get by way of, no matter prioritization.

“At least the ones at the top tend to be more likely to be acceptable and changes that we should actually make,” Settles stated. “Some of them when you look at them are like ‘yeah that’s obvious.’ Language is so expressive. There’s so many ways of saying the exact same thing that even if you’re thinking really hard about it you won’t necessarily cover all of the bases.”

The outcomes converse for themselves.

“It used to be that when we rolled out a brand new course that it took about six months or so to graduate from beta,” Settles famous. “One of the criteria of graduating from beta is that we have a fewer than a certain number of reports per number of sessions. The first two courses that we rolled out after we created this tool I think were Latin and Scottish Gaelic. Those graduated from beta in five weeks. It has significantly cut down how quickly we could deal with these reports as they came in.”

CEFR Checker

In a single quarter final yr, Duolingo used unsupervised machine studying to construct a software for figuring out the problem of any textual content for language learners. The staff used the Common European Framework of Reference (CEFR), which has a six-level scale: A1 and A2 (newbie), B1 and B2 (intermediate), and C1 and C2 (superior).

Duolingo CEFR levels

Not solely does the software classify the language degree of the textual content, it additionally judges the extent of particular person phrases and constructions. The public model solely has English and Spanish, which you’ll be able to strive your self (CEFR Checker), however internally Duolingo can be has it working for Spanish, French, Portuguese, German, and Italian.

“Our language and curriculum experts, as they’re developing the curricula, they organize vocabulary into the different levels,” Settles defined. “We’re basing this off of decades of research that has already been done on vocabulary profiles. We use that as training data. But the vast majority of work that is done in creating these vocabulary profiles is English only, because learning English is a multi-billion dollar industry whereas learning Portuguese, not so much.”

That limitation meant the staff needed to lean on its PhDs in linguistics with classroom instruction expertise who develop a variety of the course content material. These linguists put collectively profiles of round 7,000 English phrases and labeled them per the CEFR. Then the AI staff started working coaching the mannequin utilizing large quantities of textual content on the web so it could possibly study the problem of all 10 million phrases within the English language through phrase embeddings and switch studying.

CEFR checker English Spanish French German

“We invented some multilingual natural language processing ways of transfer learning,” Settles stated. “We’re essentially doing multilingual multitask transfer learning where we have mostly data in English, but we’re able to train a system that can make accurate predictions in Spanish, French, German, Italian, and Portuguese, even though we’re bootstrapping from English. It does make some mistakes. The curriculum experts in those languages can correct salient mistakes and then we retrain the model until it becomes more accurate.”

Duolingo has a Stories tab that options quick tales for testing your studying comprehension. The Stories staff makes use of CEFR Checker to check whether or not the problem degree of what they write is acceptable.

Duolingo Stories feature

“We say, ‘okay, well we need 10 more stories at this specific language level,’” von Ahn stated. “Then we have writers write them and then we check whether they’re at that language level. If they’re not, we return them to the writers and we say ‘hey this is too difficult still, you should simplify it.’”

Duolingo additionally data podcasts so you possibly can continue learning exterior of the app. The podcast staff equally makes use of CEFR Checker to ensure the script they wrote earlier than they begin recording is the precise degree of issue for a given episode. Other groups on the firm are additionally utilizing CEFR Checker and making characteristic requests to the purpose that Settles needs to return and enhance it.

Duolingo CEFR Checker article

Above you possibly can see CEFR Checker’s evaluation of this text.

What subsequent?

The most vital query I requested the duo was the one factor that Duolingo customers battle with probably the most: What order ought to I do classes in?

“We’ve explored this and we probably should continue exploring it,” von Ahn stated. “This is something that we know that a lot of people struggle with — what is the best order to do it? We’ve thought about this quite a bit and yeah, that’s something that we have used AI in the past but I don’t think we’ve ever done anything that is better than what we currently have there, which is just kind of letting people explore.”

Duolingo unlocks more durable classes primarily based on previous classes you’ve accomplished, however that’s the one steerage you get. Could AI assist you to choose what to study subsequent?

“Probably at some point because we now have the tools to start working on that,” Settles stated. “So it is in the backlog of things to put on the roadmap.”

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