In academia and research, the focus of ML is usually to build the best possible models for a given dataset. However, in practical applications, the overall performance of our application is often determined primarily by data quality, not by the model. That is why many ML practitioners advocate for Data-Centric ML approaches, where we focus on improving the data while keeping the ML model (mostly) fixed. See this great article by neptune.ai for more details on model-centric vs. data-centric ML. One of the most critical parts of data-centric ML is to monitor data quality. With ZenML’s data validators, we can check many potential data issues, such as train-test skew, training-serving skew, data drift, and more. Being aware of these issues, and having respective safety mechanisms in place, is essential when serving ML models to real users. To give one example, we can automatically check for Data Skew within our ML pipelines. Since the performance of ML models on unseen data can be unpredictable, we should always try to design our training data to match the actual environment where our model will later be deployed. The difference between those data distributions is called Training-Serving Skew. Similarly, differences in distribution between our training and testing datasets are called Train-Test Skew. In the following, we will use the open-source data monitoring tool Evidently to measure distribution differences between our datasets. See this blog post of ours that explains the Evidently integration in more detail.

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Detect Train-Test Skew

We can use Evidently, one of our data validator stack components, to check for skew between our training and test datasets. To do so, we will define a new pipeline with an Evidently step, into which we will then pass our training and test datasets. At its core, Evidently’s distribution difference calculation functions take in a reference dataset and compare it with a separate comparison dataset. These are both passed in as pandas DataFrames, though CSV inputs are also possible. ZenML implements this functionality in the form of several standardized steps along with an easy way to use the visualization tools also provided along with Evidently as ‘Dashboards’. For data distribution comparison, we can simply use the predefined step of ZenML’s Evidently integration: 
from zenml.integrations.evidently.steps import EvidentlyProfileConfig

# configure the Evidently step
evidently_profile_config = EvidentlyProfileConfig(
    profile_sections=["datadrift"]
)
We already have a standard datadrift skew or data drift step defined in ZenML, so to use this in your pipelines, simply pass the step in as required. You can see an example of this in the following pipeline initialization: 
from zenml.integrations.evidently.steps import evidently_profile_step

evidently_pipeline = digits_pipeline_with_train_test_checks(
    importer=importer(),
    trainer=svc_trainer(),
    evaluator=evaluator(),
    get_reference_data=get_reference_data(),
    skew_detector=evidently_profile_step( # here we use the pre-defined step
        step_name="evidently_skew_detector",
        config=evidently_profile_config,
    ),
)
Before we can run the pipeline, we still need to add Evidently into our ZenML MLOps stack as a data validator: 
zenml data-validator register evidently_validator --flavor=evidently
zenml stack update  -dv evidently_validator
Other ways of validating our data include the use of the following integrations: See the respective examples to get to know how they each work and what they can be used for.
To read a more detailed guide about how Data Validators function in ZenML, click here.