A streak, the daily counter many drill apps keep, counts days. A day is a container, not a unit of learning, and a counter on the outside of the container cannot see what was in it. So the gap between a long streak and what you can say is not a fault: the number is doing exactly what a number on presence does.
What a counter can count
A counter on presence goes up when you were present. Four minutes of recognising words you already knew raises it by one. Four minutes of writing sentences you had to reach for raises it by one. The two are not the same act, and the counter is not built to know that, because presence is the thing it was designed to measure and it measures it accurately.
The difference between the two acts is not a matter of degree, and it has been measured. In 2006 Roediger and Karpicke found that a test beat rereading when both groups were checked a week later, while the rereaders were the more confident of the two; Bahrick, measuring school Spanish as much as fifty years on, found that the decline levels off and that what is lost first is the route to a word rather than the word. Picking an answer asks you to recognise something already in front of you, and producing a sentence asks you to find the route, which is the costlier of the two and the one a day cannot be inspected for. The longer version of that is the post in this blog on why you forget a language.
So a day of recognition and a day of retrieval both cost you a day, both look identical on a calendar, and only one of them pays. Nothing prevents them coming apart, and once they have come apart the number keeps rising while the thing it stands for does not.
None of that is a criticism of any app. A counter on presence is a good instrument for what it measures, which is turning up, and turning up is not a trivial thing to get somebody to do. It is simply not an instrument for the other question, and it was never claimed to be.
What the missing day actually costs
Here is the part that is easy to get the wrong way round. The cost of a streak is not the day you miss. It is what the counter taught you to protect in the weeks before that.
A number you have kept for two hundred days becomes a thing with its own value, separate from the language. On a tired evening the cheapest way to keep it is the shortest available action, so the shortest available action is what you take, and the habit quietly reorganises itself around the number rather than around the language. That works, in the sense that the number survives. Then something ordinary happens, a flight or a bad week or a flat battery, and the number goes to zero. What is left holding the habit up is whatever you were doing it for underneath, and if the answer for the last two hundred days has been the number, a zero takes the answer away with it and leaves nothing underneath. The missed day cost a day. The reorganisation before it cost the reason.
Contact does not have that shape, because there is nothing to protect. Somebody asked you a question and you had to produce the answer: that is the unit, and either it happened or it did not. It cannot be performed cheaply, because the cheap version of producing a sentence is not producing a sentence. And a Tuesday you miss costs a Tuesday. There is no accumulated number to lose, so there is nothing that a missed day can invalidate, and nothing to feel you have forfeited.
This is a statement about mechanism and not a claim about what it will do for you. Nothing here keeps anybody consistent. A companion cannot make you answer, and quiet weeks are allowed: you can pause for two or four weeks, or cancel, from one screen, without writing an email or making a call. There is no course to work through and no level to pass, and the Sunday letter counts a week that has already happened rather than setting a target for the next one. That is the whole extent of it.
What follows practically is small. If you are going to measure something, measure the thing that costs you something to do: sentences you produced without a prompt in front of you, conversations where you had to choose the tense, the number of times you reached for a word and got there. Those are harder to count than days, which is exactly why days get counted instead. And if a number is what gets you to open something on a Tuesday evening, keep it. Just do not read it as an account of what your language is doing, because it is an account of your attendance, and the two only agree by coincidence.
Sources: Roediger and Karpicke, Test-Enhanced Learning, Psychological Science 17(3), 2006. Bahrick, Semantic memory content in permastore, Journal of Experimental Psychology: General 113(1), 1984.
