Convenience sampling

Sampling in Python

James Chapman

Curriculum Manager, DataCamp

The Literary Digest election prediction

A Literary Digest front page from 1936 showing a headline of election predictions. Landon was expected to get 1.3 million votes, and Roosevelt was expected to get just under 1 million votes.

Prediction: Landon gets 57%; Roosevelt gets 43%
Actual results: Landon got 38%; Roosevelt got 62%
Sample not representative of population, causing sample bias
Collecting data by the easiest method is called convenience sampling

Finding the mean age of French people

A photo of Disneyland Paris.

Survey 10 people at Disneyland Paris
Mean age of 24.6 years
Will this be a good estimate for all of France?

¹ Image by Sean MacEntee

How accurate was the survey?

Year	Average French Age
1975	31.6
1985	33.6
1995	36.2
2005	38.9
2015	41.2

24.6 years is a poor estimate
People who visit Disneyland aren't representative of the whole population

Convenience sampling coffee ratings

coffee_ratings["total_cup_points"].mean()

82.15120328849028

coffee_ratings_first10 = coffee_ratings.head(10)

coffee_ratings_first10["total_cup_points"].mean()

89.1

Visualizing selection bias

import matplotlib.pyplot as plt
import numpy as np
coffee_ratings["total_cup_points"].hist(bins=np.arange(59, 93, 2))
plt.show()

coffee_ratings_first10["total_cup_points"].hist(bins=np.arange(59, 93, 2))
plt.show()

Distribution of a population and of a convenience sample

Population: A histogram of cup points from the population.

Convenience sample: A histogram of cup points from the sample.

Visualizing selection bias for a random sample

coffee_sample = coffee_ratings.sample(n=10)
coffee_sample["total_cup_points"].hist(bins=np.arange(59, 93, 2))
plt.show()

Distribution of a population and of a simple random sample

Population: A histogram of cup points from the population.

Random Sample: A histogram of cup points from a random sample.

Let's practice!

Sampling in Python