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MIT's "Machine Learning" lecture notes

PDF: https://introml.mit.edu/_static/spring24/LectureNotes/6_390_lecture_notes_spring24.pdf

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Use Python to turn messy data into valuable insights!

Here are the main functions you need to know:

1. 𝗱𝗿𝗼𝗽𝗻𝗮(): Clean up your dataset by removing missing values. Use df.dropna() to eliminate rows or columns with NaNs and keep your data clean.

2. 𝗳𝗶𝗹𝗹𝗻𝗮(): Replace missing values with a specified value or method. With the help of df.fillna(value) you maintain data integrity without losing valuable information.

3. 𝗱𝗿𝗼𝗽_𝗱𝘂𝗽𝗹𝗶𝗰𝗮𝘁𝗲𝘀(): Ensure your data is unique and accurate. Use df.drop_duplicates() to remove duplicate rows and avoid skewing your analysis by aggregating redundant data.

4. 𝗿𝗲𝗽𝗹𝗮𝗰𝗲(): Substitute specific values throughout your dataset. The function df.replace(to_replace, value) allows for efficient correction of errors and standardization of data.

5. 𝗮𝘀𝘁𝘆𝗽𝗲(): Convert data types for consistency and accuracy. Use the cast function df['column'].astype(dtype) to ensure your data columns are in the correct format you need for your analysis.

6. 𝗮𝗽𝗽𝗹𝘆(): Apply custom functions to your data. df['column'].apply(func) lets you perform complex transformations and calculations. It works with both standard and lambda functions.

7. 𝘀𝘁𝗿.𝘀𝘁𝗿𝗶𝗽(): Clean up text data by removing leading and trailing whitespace. Using df['column'].str.strip() helps you to avoid hard-to-spot errors in string comparisons.

8. 𝘃𝗮𝗹𝘂𝗲_𝗰𝗼𝘂𝗻𝘁𝘀(): Get a quick summary of the frequency of values in a column. df['column'].value_counts() helps you understand the distribution of your data.

9. 𝗽𝗱.𝘁𝗼_𝗱𝗮𝘁𝗲𝘁𝗶𝗺𝗲(): Convert strings to datetime objects for accurate date and time manipulation. For time series analysis the use of pd.to_datetime(df['column']) will often be one of your first steps in data preparation.

10. 𝗴𝗿𝗼𝘂𝗽𝗯𝘆(): Aggregates data based on specific columns. Use df.groupby('column') to perform operations like sum, mean, or count on grouped data.

Learn to use these Python functions, to be able to transform a pile of messy data into the starting point of an impactful analysis.

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🔰 Python Toolkit for Data Analysis

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Python project-based interview questions for a data analyst role, along with tips and sample answers [Part-1]

1. Data Cleaning and Preprocessing
   - Question: Can you walk me through the data cleaning process you followed in a Python-based project?
   - Answer: In my project, I used Pandas for data manipulation. First, I handled missing values by imputing them with the median for numerical columns and the most frequent value for categorical columns using fillna(). I also removed outliers by setting a threshold based on the interquartile range (IQR). Additionally, I standardized numerical columns using StandardScaler from Scikit-learn and performed one-hot encoding for categorical variables using Pandas' get_dummies() function.
   - Tip: Mention specific functions you used, like dropna(), fillna(), apply(), or replace(), and explain your rationale for selecting each method.

2. Exploratory Data Analysis (EDA)
   - Question: How did you perform EDA in a Python project? What tools did you use?
   - Answer: I used Pandas for data exploration, generating summary statistics with describe() and checking for correlations with corr(). For visualization, I used Matplotlib and Seaborn to create histograms, scatter plots, and box plots. For instance, I used sns.pairplot() to visually assess relationships between numerical features, which helped me detect potential multicollinearity. Additionally, I applied pivot tables to analyze key metrics by different categorical variables.
   - Tip: Focus on how you used visualization tools like Matplotlib, Seaborn, or Plotly, and mention any specific insights you gained from EDA (e.g., data distributions, relationships, outliers).

3. Pandas Operations
   - Question: Can you explain a situation where you had to manipulate a large dataset in Python using Pandas?
   - Answer: In a project, I worked with a dataset containing over a million rows. I optimized my operations by using vectorized operations instead of Python loops. For example, I used apply() with a lambda function to transform a column, and groupby() to aggregate data by multiple dimensions efficiently. I also leveraged merge() to join datasets on common keys.
   - Tip: Emphasize your understanding of efficient data manipulation with Pandas, mentioning functions like groupby(), merge(), concat(), or pivot().

4. Data Visualization
   - Question: How do you create visualizations in Python to communicate insights from data?
   - Answer: I primarily use Matplotlib and Seaborn for static plots and Plotly for interactive dashboards. For example, in one project, I used sns.heatmap() to visualize the correlation matrix and sns.barplot() for comparing categorical data. For time-series data, I used Matplotlib to create line plots that displayed trends over time. When presenting the results, I tailored visualizations to the audience, ensuring clarity and simplicity.
   - Tip: Mention the specific plots you created and how you customized them (e.g., adding labels, noscripts, adjusting axis scales). Highlight the importance of clear communication through visualization.

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