# Missing Data

This lesson focuses on missing data by explaining some techniques to clean such data from our dataset.

We'll cover the following

## What is missing data? #

So, you have read in your data only to discover that some values are missing! What do you do?

First, you should try to understand why your data points are missing. Are they missing at random or not? Data that is missing at random could be removed with a large enough dataset. If your data is missing for a reason, perhaps missing means zero? Or is it a strong signal of a sensor malfunction? These non-random missing values should be fixed or leveraged. For example, set them to zero if they should be zero.

Second, you should consider how much data you have relative to how many rows have missing data. If you have 1 million data points with 10 points missing at random, you are probably okay to drop those rows. On the other hand, if you have 300 data points with 100 are missing, then you should probably be hesitant about removing 1/3 of your data. Assuming you can’t find an underlying reason for the missing data, how do you handle them?

## Filling missing data #

There are a few ways to fill in missing values.

### Using a statistical value #

Fill in a column’s missing values with a statistical value such as the mean, median, or mode. This has the advantage of being simple and easy to do but also introduces a smaller variance within your column than otherwise would not be present. High variance within a column is usually beneficial (assuming it is real) to a machine learning model. Also, it might not make a lot of sense. For example, imagine a data set of heights and weights with missing weights. You then fill all the missing weights with the average regardless of height. We know that height and weight are correlated, but we are not taking advantage of that knowledge.

Let’s look at a simple example of our first idea of filling in missing values with a statistical value.

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