# Statistics

You're an intern at a rapidly growing tech start-up. They've recently developed a web application for data analysis, and a crucial part of it is calculating statistics. As they've been growing, the application's code base has become quite large, and it's time to start organizing things better. Your task for the week is to create a Python module called `statistics_calculator` that contains functions to calculate the mean, median, mode, and standard deviation of a list of numbers.
You should structure your code as follows: Create separate files for each of the statistics, with their respective calculation function. These files should be:
• `mean.py`: Contains a function `calculate_mean` to calculate the mean.
• `median.py`: Contains a function `calculate_median` to calculate the median.
• `mode.py`: Contains a function `calculate_mode` to calculate the mode (in case there are several most frequent numbers the function should return the smallest one).
• `std_dev.py`: Contains a function `calculate_std_dev` to calculate the standard deviation.
Each of these files should be within a directory named `statistics_calculator`. Finally, import all these functions in the `statistics_calculator/__init__.py` file so that other parts of the application can access these functions without additional imports.
The input and output are handled automatically by the `main.py` module.
Your code structure should look like this:
``````statistics_calculator/
├── __init__.py
├── mean.py
├── median.py
├── mode.py
└── std_dev.py``````
 Input Output ```a=[2, 2, 3, 5, 7, 5, 7, 5, 2, 2]; print(calculate_mean(a)); print(calculate_median(a)); print(calculate_mode(a)); print(calculate_std_dev(a))``` 4.0 4.0 2 2.05480467

#### Constraints

Time limit: 2 seconds

Memory limit: 512 MB

Output limit: 1 MB