Santiago Profile picture
17 Feb, 9 tweets, 2 min read
Imagine a model to predict customers who will unsubscribe from your service.

You want to incentivize them with $10 because they will cost you $100 if they churn.

Look at the attached confusion matrix showing that the model is only 77% accurate.

Is this model good enough?
I love this question because it puts a couple of things in perspective:

1. A model that doesn't look too good by the numbers.

2. A business case that can use a less-than-ideal solution to solve the problem.
There's only one question about this problem: how many people will not churn if they get the incentive?

We don't know, but we can play out different scenarios and see what happens.
Assume everyone who takes the incentive sticks around.

That means that we will be spending $190 on incentives.

Churn will cost us $1,300 (13 people will churn — the false negatives.)

Total cost: $1,490.

If we did nothing, we would have $2,200 in costs.

The model is useful!
Assume that half the people who take the incentive stay.

We will still spend $190 on incentives, but churn will be ~(13 + 4) * $100 = $1,700.

The total cost is $1,890 which is still less than $2,200.

Model is still useful with the incentive at a 50% success rate.
This model wouldn't make sense if we can only get a single person (or none of them) to stay after getting the incentive.

In every other case, the model will be useful.
Here is the summary of the story:

In a business context, performance metrics aren't the whole story. The economy of things plays a more prominent role.

(And of course, in this example we are assuming we created the model for free... but it's just an example.)
@AlejandroPiad I remember we had a conversation a while ago about measuring whether a model is useful or not. We discussed ROI that day.

This model is one example.
Even better yet, assuming that the incentive is 100% effective, then giving $10 to everyone will save more money than using the model in the first place.

It all comes down to the assumptions we make about the effectiveness of that incentive.

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More from @svpino

18 Feb
You gotta think about this one carefully!

Imagine you go to the doctor and get tested for a rare disease (only 1 in 10,000 people get it.)

The test is 99% effective in detecting both sick and healthy people.

Your test comes back positive.

Are you really sick? Explain below 👇
The most complete answer from every reply so far is from Dr. Lena. Thanks for taking the time and going through it!

You can get the answer using Bayes' theorem, but let's try to come up with it in a different —maybe more intuitive— way.

👇 Image
Read 9 tweets
16 Feb
Imagine that you ask a yes/no question to 1,000 people, and each person answers correctly 51% of the time.

You count the different answers and pick the most common one.

How likely are you to end up with the correct answer?

🧵👇
Don't feel bad if you think it's 51% — We all did!

If every person answers independently from the rest, you'll end up with the correct answer ~75% of the time.

And if you ask 10,000 people, the chance of getting it right goes up to ~97%!

Mind-blowing, right?

(2 / 6)
If you care about the math behind this, take a look at the attached expression. (But it doesn't matter if you don't.)

This is what's important:

The law of large numbers ensures that we get more correct answers as we ask more people.

(3 / 6)
Read 7 tweets
15 Feb
I have not seen any proof that Twitter "kills your content" if you include links to your tweets.

Here is the result of a very unscientific experiment: comparing my top 10 tweets with and without links.

If you have something concrete, please let me know.
This is anecdotal evidence at best.

It doesn't prove that Twitter doesn't mess with your links, but it does suggest that —if anything is going on— it is much more subtle than what some believe.

I haven't found any documentation either.
This is what I do know:

Breaking the links that you add to your tweets is self-serving: it makes it worse for the people who follow you. They can't just click to get the content.

I can't see how this will make your content better in any way.
Read 4 tweets
14 Feb
A collection of the most interesting threads I've written about machine learning.

🧵 x 🧵
Read 13 tweets
13 Feb
It takes a single picture of an animal for my son to start recognizing it everywhere.

Neural networks aren't as good as we are, but they are good enough to be competitive.

This is a thread about neural networks and bunnies.

🧵👇
A few days ago, I discussed how networks identify patterns and use them to extract meaning from images.

Let's start this thread right from where we ended that conversation.



(2 / 16)
Let's assume we use these four pictures to train a neural network. We tell it that they all contain a bunny 🐇.

Our hope is for the network to learn features that are common to these images.

(3 / 16)
Read 17 tweets
11 Feb
Today let's talk about why we keep "splitting the data" into different sets.

Besides machine learning people being quirky, what else is going on here?

Grab your coffee ☕️, and let's do it!

🧵👇
Imagine you are teaching a class.

Your students are getting ready for the exam, and you give them 100 answered questions, so they prepare.

You now need to design the exam.

What's the best way to evaluate the students?

(2 / 19)
If you evaluate the students on the same questions you gave them to prepare, you'll reward those who just memorized the questions.

That won't give you a good measure of how much they learned.

😑

(3 / 19)
Read 21 tweets

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