## Imputation of Missing Data in Python: Techniques, Importance, and Applications

Missing data is a common occurrence in data analysis, and it can be a challenging hurdle for data scientists to overcome. Missing data refers to the absence of values in a dataset that can affect the accuracy of machine learning models and other analytical techniques.

In this article, we will explore the top techniques for imputing missing data using Python and discuss the importance of imputation in data analysis. As datasets grow larger and more complex, it is inevitable that some of the data will be missing or null. Missing data can occur due to multiple reasons such as technical failures, data entry errors, or respondents skipping questions during surveys.

Whatever the cause, missing data can significantly impact data analysis and modeling. Unaddressed missing data can lead to biased effect estimation, inefficient model performance, and hazardous effects.

Therefore, it is important to have robust imputation techniques to deal with missing data.

## Techniques for Imputation

Imputation is the process of replacing missing data with estimated values. There are various techniques available for imputing missing data in a dataset.

### Some of the popular techniques are:

### Imputation using Mean

### Imputation using Median

### KNN Imputation

### Imputation using Mean

Imputation using Mean technique replaces missing values with the mean of the remaining values in that column. This technique is commonly used when the missing data is missing completely at random (MCAR).

To implement the mean imputation in Python, one can use the pandas.read_csv() function to load the dataset, followed by the isnull() function to check the missing values. Finally, the mean() function can be used to calculate the mean of the remaining values in the column.

### Imputation using Median

Imputation using Median is similar to imputation using mean, except that instead of using the mean, we use the median of the remaining data as the estimated value. This technique is used when the data is skewed or there are outliers.

We can implement the median imputation by loading the dataset and checking for missing values using the isnull() function, and then using the median() function to calculate the median of the remaining data in the column.

### KNN Imputation

KNN Imputation is a more advanced technique that involves using a machine learning algorithm known as K-Nearest-Neighbors (KNN). This imputation technique is used when there is a high correlation between columns in the dataset.

KNN imputation involves finding the K-nearest-neighbors to a missing value and using their values to estimate the missing value.

## Importance of Imputation

Imputation is an essential process in data analysis as it ensures the efficiency of machine learning models, provides balanced data distribution, and prevents hazardous effects.

### Efficient Machine Learning Model

Machine learning models require complete data to generate accurate predictions. Missing data affects the performance of machine learning models and creates a biased representation of the real-world scenario.

Imputing the missing data ensures that models are trained with complete data for better accuracy and efficient predictions.

### Balanced Data Distribution

Data analysis largely depends on data distribution. Missing data negatively impacts data distribution and makes it biased, leading to inaccurate estimates and predictions.

Imputing missing data ensures that the data distribution is correctly represented leading to unbiased, accurate and reliable conclusions from data.

### Hazardous Effects

Null values in a dataset cannot be ignored. They can lead to hazardous effects such as incorrect predictions, biased decisions, and compromised models.

For example, if null values are not addressed in medical data, it can lead to hazardous effects such as inaccurate diagnoses and treatments. Similarly, missing data in financial data could lead to fraudulent activities, leading to financial losses for organizations.

## Conclusion

In conclusion, missing data is a common issue in data analysis, and robust imputation techniques are essential in ensuring accurate data analysis, reliable and efficient predictions. In this article, we have covered some popular techniques such as mean imputation, median imputation, and KNN imputation.

We have also highlighted the importance of imputation in data analysis, particularly for efficient machine learning models and unbiased data distribution. Verification of Missing Data: Meaning and Importance

Verification of missing data refers to the process of validating the presence of any missing values in a dataset.

Before applying imputation techniques, it is crucial to check whether missing data exists in the dataset. Verification can be done before and after imputation to ensure the accuracy of the data.

In this section, we will discuss both types of verification and the significance of verifying missing data in data analysis.

### Prior to Imputation

Verification of missing data prior to imputation is vital as it helps to identify any missing data in the dataset. It is crucial for any data analyst or machine learning model developer to identify which columns in the dataset are missing values.

Verification is done using the isnull() function of pandas, which is used to check for missing values in the dataset.

The isnull() function returns a Boolean value to specify whether a given cell in the dataset contains any missing value.

On implementing the isnull() function in the dataset, it searches for the missing values and indicates their presence with a True value. Therefore, to verify missing data before imputation, one requires to apply isnull() on the dataset and check if the output contains any True values.

### After Imputation

Verification of missing data after imputation is as important as before imputation as it helps to ensure the accuracy of the estimated values used in imputation. It means that the missing values were filled using the right techniques based on the correct data.

Verification after imputation is done by calculating the count of null or missing values in the dataset using the isnull() function. If the verified dataset still contains any missing values after imputation, this may indicate that the imputation technique used was not appropriate or that there was a problem with the dataset.

At this stage, data analysts are required to apply the appropriate imputation techniques to replace any remaining missing values.

## Imputation Using Mean and Median

Imputation using mean and median are two common techniques that are used to replace missing data in a dataset. Both techniques used the central tendency of the dataset to calculate the estimated value to replace the missing one.

In the following sections, we will discuss the two techniques in detail, including their benefits and limitations.

### Mean Imputation

Mean imputation is a simple and straightforward technique used in imputation where the average value of a given data column is used as the reference to fill a missing value. The mean() function in Pandas can be used to implement the mean imputation.

Through Mean imputation, we can maintain the original data distribution since we are only filling missing values with the mean value of the column. Mean imputation is recommended when data is missing completely at random (MCAR), but it should be approached with caution for skewed datasets that can potentially result in outlier imputation.

For example, let us consider a dataset with missing data as shown below:

column1 | column2 |
---|---|

100 | 50 |

75 | NaN |

80 | 30 |

NaN | 45 |

90 | 50 |

To apply mean imputation, we first calculate the mean value for the column with missing values (column2) and replace the missing value with this average value. `df['column2'] = df['column2'].fillna(df['column2'].mean())`

After implementing the above code, the updated dataset will be:

column1 | column2 |
---|---|

100 | 50 |

75 | 44 |

80 | 30 |

79 |
45 |

90 | 50 |

### Median Imputation

Median imputation is a technique that is used to fill missing values with the median value of the data set. This imputation technique is useful in scenarios where data has a skewed distribution with outlier values, as median is less sensitive to outliers compared to mean.

To apply median imputation, we first calculate the median value for the column with missing values and then replace the missing value with this median. The median() function in Pandas can be used to calculate the median value.

For example, if we consider the above dataset, we will replace the missing value in the second row using the median function:

`df['column2'] = df['column2'].fillna(df['column2'].median())`

After implementing the above code, the updated dataset will be:

column1 | column2 |
---|---|

100 | 50 |

75 | 47.5 |

80 | 30 |

47.5 |
45 |

90 | 50 |

## Conclusion

Verification of missing data prior to and after imputation is crucial to ensuring data accuracy in analyses. Verification helps to identify missing data columns and ensures that the imputed values remain accurate even after replacing missing values.

Mean and median imputations are common methods used in the replacement of missing values. Mean imputation is best suited for MCAR data while median imputation is useful for a skewed dataset with outliers.

As data analysts, we need to take due care when handling missing data.

## KNN Imputation:

## Implementation, Data Types, and Output

KNN imputation is a powerful technique used in imputing missing data values in a dataset. KNN imputation estimates missing data values based on their relationship with the values in the same column.

In this section, we will explore how to implement KNN imputation, how to convert data types, and analyze the output of the KNN imputation.

### Implementation

KNN imputation involves using the K-Nearest-Neighbors algorithm to estimate missing values. This algorithm groups similar data points together creating a cluster.

The missing data point is estimated based on a weighted average of its kin the data point’s K-nearest-neighbors.

To implement KNN in Python, we load the dataset using pandas.read_csv() function, check for missing values using isnull() and then apply the KNN method from the fancy impute python package.

The following code snippet shows how KNN imputation is done in Python,

```
from fancyimpute import KNN
import pandas as pd
#load dataset
df = pd.read_csv('data.csv')
#check for missing values
print(df.isnull().sum())
#impute missing values using KNN
df_imputed_KNN = KNN(k=10).fit_transform(df)
```

In the above code, we import the KNN function from the fancy impute package, load the dataset, check for missing values using isnull(), and then impute the missing values using the KNN function with k=10.

### Converting Data Types

Before applying the KNN imputation technique, it’s crucial to ensure that all data types are uniform. KNN imputation works best for numerical data, and hence categorical data must be converted into numerical data.

The conversion of categorical data into numerical data is performed using a categorical code. Therefore, the category is assigned a code that reflects its position in a set of categories.

The pandas package provides a category data type to facilitate this conversion.

For example, let us consider a dataset that has both numeric and categorical data as shown below:

column1 | column2 |
---|---|

100 | X |

75 | Y |

80 | Z |

NaN | Y |

90 | X |

To convert the categorical data in column2 to a numerical format, we can use the categorical() method and then retrieve the codes:

```
df['column2'] = df['column2'].astype('category')
df[column2] = df[column2].cat.codes
```

After implementing the above code, the updated dataset will be:

column1 | column2 |
---|---|

100 | 0 |

75 | 1 |

80 | 2 |

NaN | 1 |

90 | 0 |

### Output of Imputation

The output of KNN imputation includes replaced missing values, the elapsed time in the imputation process, and the count of null values in the imputed dataset. Since KNN imputation involves finding the K-nearest neighbors of the missing data point, its computational efficiency is sensitive to the number of neighbors k.

However, smaller values of k will result in increased variability in the estimates while larger k values will reduce the variability but increase the computational time. After applying KNN imputation, it is crucial to verify the output data.

One way to achieve this is to check the count of null values. The dataset should have filled in all the missing values.

Another way to verify the output is to determine the elapsed time. The elapsed time can provide information about the computational complexity and the amount of time required to complete the imputation.

For example, we can estimate the computational time using the following code:

```
from time import time
#load dataset
df = pd.read_csv('data.csv')
#check for missing values
print(df.isnull().sum())
#impute missing values using KNN
start_time = time()
df_imputed_knn = KNN(k=10).fit_transform(df)
elapsed_time = time() - start_time
print('Elapsed Time: ', elapsed_time, 'Seconds')
```

## Conclusion

KNN imputation is a great technique for data imputation that benefits in presenting a complete dataset. Converting data types prior to KNN imputation is vital.

Due diligence is also required to verify the output after imputation. In this article, we have explored how to implement KNN imputation, how to convert data types into uniform formats, and analyze the output of the imputation.

With the combinations of the three techniques discussed in this article, an accurate and efficient data analysis and machine learning would be possible. In summary, dealing with missing data is essential for accurate data analysis models, and imputation is the process of replacing missing values with estimated values.

Mean imputation, median imputation, and KNN imputation are popular techniques for imputing missing data. Verification of missing data is an important step in data analysis, both before and after imputation.

Additionally, it is important to consider data types and verify the output of the imputation process. In conclusion, data analysts must understand the different techniques for handling missing data, including their implementation, verification, and output, to ensure the efficient and accurate analysis of large datasets.

By employing these techniques, analysts can provide reliable, unbiased, and efficient data-driven results that can inform decision-making for businesses and organizations.