Code to calculate and plot the z-score

zscore.py

import numpy as np
import matplotlib.pyplot as plt
import scipy.stats as scs

def z_val(sig_level=0.05, two_tailed=True):
    """Returns the z value for a given significance level"""
    z_dist = scs.norm()
    if two_tailed:
        sig_level = sig_level/2
        area = 1 - sig_level
    else:
        area = 1 - sig_level

    z = z_dist.ppf(area)

    return z
  
  
def zplot(area=0.95, two_tailed=True, align_right=False):
    """Plots a z distribution with common annotations

    Example:
        zplot(area=0.95)

        zplot(area=0.80, two_tailed=False, align_right=True)

    Parameters:
        area (float): The area under the standard normal distribution curve.
        align (str): The area under the curve can be aligned to the center
            (default) or to the left.

    Returns:
        None: A plot of the normal distribution with annotations showing the
        area under the curve and the boundaries of the area.
    """
    # create plot object
    fig = plt.figure(figsize=(12, 6))
    ax = fig.subplots()
    # create normal distribution
    norm = scs.norm()
    # create data points to plot
    x = np.linspace(-5, 5, 1000)
    y = norm.pdf(x)

    ax.plot(x, y)

    # code to fill areas for two-tailed tests
    if two_tailed:
        left = norm.ppf(0.5 - area / 2)
        right = norm.ppf(0.5 + area / 2)
        ax.vlines(right, 0, norm.pdf(right), color='grey', linestyle='--')
        ax.vlines(left, 0, norm.pdf(left), color='grey', linestyle='--')

        ax.fill_between(x, 0, y, color='grey', alpha='0.25',
                        where=(x > left) & (x < right))
        plt.xlabel('z')
        plt.ylabel('PDF')
        plt.text(left, norm.pdf(left), "z = {0:.3f}".format(left), fontsize=12,
                 rotation=90, va="bottom", ha="right")
        plt.text(right, norm.pdf(right), "z = {0:.3f}".format(right),
                 fontsize=12, rotation=90, va="bottom", ha="left")
    # for one-tailed tests
    else:
        # align the area to the right
        if align_right:
            left = norm.ppf(1-area)
            ax.vlines(left, 0, norm.pdf(left), color='grey', linestyle='--')
            ax.fill_between(x, 0, y, color='grey', alpha='0.25',
                            where=x > left)
            plt.text(left, norm.pdf(left), "z = {0:.3f}".format(left),
                     fontsize=12, rotation=90, va="bottom", ha="right")
        # align the area to the left
        else:
            right = norm.ppf(area)
            ax.vlines(right, 0, norm.pdf(right), color='grey', linestyle='--')
            ax.fill_between(x, 0, y, color='grey', alpha='0.25',
                            where=x < right)
            plt.text(right, norm.pdf(right), "z = {0:.3f}".format(right),
                     fontsize=12, rotation=90, va="bottom", ha="left")

    # annotate the shaded area
    plt.text(0, 0.1, "shaded area = {0:.3f}".format(area), fontsize=12,
             ha='center')
    # axis labels
    plt.xlabel('z')
    plt.ylabel('PDF')

    plt.show()