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Why do we need to care about Retraining?

Shikha Mohanty | 14th Jan, 2023

In our previous blog, we introduced you to the wonderful universe of ML and dug deep into the various steps involved in building and deploying the model. But there is one very integral step that one needs to take to ensure the longevity and relevance of the model and that is Model Retraining, which is a crucial aspect in the life cycle of a model.

Let’s understand this better with an example. IMDb is one of the world’s largest movie databases and is a go-to website for many of us for movie recommendations (What would we do without it, right? XD). Let’s say for better user experience, IMDb wants to build a machine learning model which predicts box office outcome of a movie before its release based on inputs such as date of release, genre, star cast, production house, director, theme, editing and effects used, content rating (age group rating), music composer, region and budget and it will help predict whether the movie will be a box office hit or not. To do this, the model is first trained on the movie data of three years, say 2015 - 2018 and then it was deployed, and voila! It gave tremendously accurate results and everyone lived happily ever after. Or at least that’s what we all would like to believe, right?

However, the success of the model is short lived- It was noticed that around late 2020, a data shift occurs as audience preferences change, which results in the degradation of the model. The precision and accuracy rate of the model deteriorates and an overall downfall in the performance of the model is observed. But why did this happen in the first place? Well, the reason is that the data we used to train our model is now outdated.

So, what next? Cue, model retraining

But First, Let Me #EXPLAIN What is Model Retraining?

Model retraining is the process of updating the data of any deployed model. The updated data is used to retrain the model and keep the model up to date. It ensures the accuracy rate and precision rate of the model are not compromised, and quality service is ensured. However, model retraining doesn't change the model's algorithm; it simply means updating the model's dataset to get accurate results even after a long time.

6 Reasons Why ▶ Retraining is Important

The decay in model performance is generally due to reasons like data drifts, label drifts, and concept drifts. To accommodate these changes, retraining is essential. Other reasons include the occurrence of unlikely scenarios, the addition of data, or an increase in the output labels. Let’s talk about all of these one by one:

1. Data Drift: In simple words, a data shift is a change in the dataset,overtime, that is initially used to train the model. This change in dataset values leads to inaccurate predictions of the model. The initial data used to train the data is called source distribution, and the data that the model actually works on is the target distribution. Coming back to our previous IMDb example, the model was trained using data collected in the years 2015 - 2018 and it is inevitable that trends and preferences would have changed significantly since then, which would lead to a data shift. Hence, it becomes crucial to collect fresh data and update our model accordingly.

2. Concept Drift: Concept drift refers to a phenomenon where the relationship between the inputs and outputs change over time. In the context of our example, say the audience's tastes evolve over time. Previously having a big actor was enough to guarantee the success of a movie (and since this was a clear pattern in the training data, this is also what our model learnt). But now, people are giving more preference to storyline as compared to actors and as a result, movies with just popular artists but weak storylines are now getting flopped. This necessitates a need to retrain the model as the underlying relationship between the input features and resultant output has changed and the model must be retrained to the new reality.

3. Label Shift: Label shift occurs when the output distribution changes while the input distribution and their relationship remains the same. These scenarios typically happen when the underlying data undergoes a systematic shift. Let’s understand this with our movie example. All of us have seen the meteoric rise of OTT platforms in recent years. This rise has resulted in a negative impact on box office collections as people are now okay to wait for the OTT release as compared to spending on movie tickets. What it means is that the box office collections have reduced across the board. While there might not be any change in which movies are becoming a hit, the hit movies may be earning lower collections as compared to earlier. Thus, the model needs to be retrained to learn the new output distribution.

4. Edge Cases: Edge cases are the occurrence of rare yet possible scenarios. Talking in quantitative terms, if there are events with a 99% possibility, then an edge case is the occurrence of an event with that rare 1% possibility. Let's talk about our example. In this case, let's say we have an actor who only works with big production houses, but in a particular case, the script of the movie attracts him, and he ends up working with a small local production house. Now, this is an edge case for our model. Another instance of an edge case is where a less famous actor has a big break and ends up working in a high-budget film with a very famous director and producer, contrary to his previous low-budget movie castings. While we collect the dataset and are provisioning our model, we might not consider these cases, but if such a case happens, we need to collect such data intently and retrain our model to include these cases as well.

5. Enabling new use cases: Retraining is triggered in these situations where a new class has been added to the output. For instance, previously movies were being classified amongst these three categories: Flop, Average and Hit. However, a new category needs to be added called Blockbuster Hit. So, naturally the model needs to be retrained to incorporate this new class.

6. Feature change: The factors in deciding a certain outcome keeps changing with time. Let's say with all the input data earlier described, we notice that supporting cast plays a key role in determining success of a movie and want to add that also as an input feature. This requires us to retrain the model with the new set of input features. Adding the data in the dataset will be easy, but then model retraining becomes a crucial step.

The above drifts are some of the most common issues encountered by ML practitioners and we at UpTrain AI are building a smart and automated pipeline to continuously monitor these issues and solve them by retraining models so that they can meaningfully impact your business metrics. Try out our open-source repo

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