How-to: clean a dataset

Cleaning a dataset involves handling missing values, correcting errors, and preparing the data for analysis. Here are common steps to clean a dataset using Python and pandas:

1. Identify Missing Values:
   • Use df.isnull() to identify missing values in the dataset.
   • Use df.isnull().sum() to get the count of missing values in each column.
2. Handle Missing Values:
   • Decide on a strategy for handling missing values:
     • Remove rows with missing values using df.dropna().
     • Fill missing values with a specific value using df.fillna(value) or fill forward/backward using df.fillna(method='ffill'/'bfill').
3. Remove Duplicates:
   • Use df.duplicated() to identify duplicate rows. You can use parameter keep=False to see all occurrences of a duplicated record, otherwise, by default, the first occurrence of each duplicated row is marked as False, and subsequent occurrences are marked as True.
   • Use df.drop_duplicates() to remove duplicate rows.
4. Correct Data Types:
   • Ensure that columns have the correct data types (e.g., convert string columns to datetime using pd.to_datetime or df.astype).
5. Correct Errors:
   • Identify and correct any errors or inconsistencies in the data.
6. Standardize/Normalize Data:
   • Standardize or normalize numerical data if needed using methods like Min-Max scaling or Z-score normalization.
7. Handle Outliers:
   • Identify and handle outliers in numerical data. You can use statistical methods or visualization techniques to detect outliers.
8. Rename Columns:
   • Rename columns for clarity if necessary using df.rename(columns={'old_name': 'new_name'}).
9. Remove Unnecessary Columns:
   • Remove columns that are not relevant for analysis using df.drop(columns=['col1', 'col2']).
10. Ensure Consistent Casing:
    • Standardize the casing of text data if needed (e.g., convert all text to lowercase using df['column'].str.lower()).
11. Check for Inconsistencies:
    • Check for inconsistent values in categorical columns and standardize if necessary.
12. Impute or Remove Irrelevant Data:
    • If data is missing or irrelevant, consider imputing values or removing the entire row/column.
13. Check for Data Integrity:
    • Ensure that the data is consistent and adheres to the expected business rules.
14. Reindex Data:
    • If needed, reset or modify the index using df.reset_index() or df.set_index('new_index').
15. Save Cleaned Dataset:
    • Save the cleaned dataset using df.to_csv('cleaned_data.csv') or a similar method.

Always document the steps taken during the cleaning process for transparency and reproducibility. Additionally, it’s crucial to thoroughly understand the context of the data and the goals of your analysis when making decisions during the cleaning process.