# When to Use a Cumulative Frequency Graph

- December 29, 2018
- Key Terms: cumulative graph, pandas

#### Import Modules¶

```
import pandas as pd
import matplotlib.pyplot as plt
import seaborn as sns
% matplotlib inline
```

#### Set Visualization Styles¶

```
sns.set_style("darkgrid")
sns.set_context("talk")
```

### Example: College Graduation Rates¶

I generate fictional data for the count of students who graduated college in years after having the same start date.

I assign the variable `count_students_at_class_start`

as the total number of students who started college at the same time, yet not all these students graduated.

```
count_students_at_class_start = 6080
graduation_data = {'years_after_starting_college': [2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7],
'count_students_graduated': [5, 50, 150, 3380, 760, 340, 115, 80, 40, 10]}
df = pd.DataFrame(data=graduation_data)
```

Preview the data in `df`

.

```
df.head()
```

Initially, we can plot this data as a bar graph for the count of students who graduated in each half-year interval.

```
df.plot(kind='bar', x='years_after_starting_college', y='count_students_graduated',
figsize=(10, 8), legend=False, color='powderblue', rot=0);
plt.title("Count of Students Who Graduated in $X$ Years", y=1.01, fontsize=20)
plt.ylabel("count of students graduated", labelpad=15)
plt.xlabel("years after starting college", labelpad=15);
```

I find this visualization a bit confusing. With each bar, it's tough to understand the significance. So almost 3,000 students graduated in 4 years...is that good? How many students started 4 years ago? What percent of students is this from that start class?

I can answer these questions with a cumulative frequency graph. I must *cumulatively* add up the count of students that graduated - a term often called a **running total**.

In pandas, there's a `cumsum()`

method that returns a cumulative sum over a column. I'll apply that to the `count_students_graduated`

column.

```
df['count_students_graduated_running_total'] = df['count_students_graduated'].cumsum()
```

View our new `count_students_graduated_running_total`

column.

```
df
```

I'd like to plot this new column as a line graph over time. This is a cumulative graph.

```
df.plot(x='years_after_starting_college', y='count_students_graduated_running_total', kind='line',
figsize=(10, 8), legend=False, style='yo-', label="count students graduated running total")
plt.axhline(y=count_students_at_class_start, color='green', linestyle='--', label='count students at initial start')
plt.title("Running Total of Students Who Graduated in $X$ Years\nFrom Same Start Class", y=1.01, fontsize=20)
plt.ylabel("running total of students graduated", labelpad=15)
plt.xlabel("years after starting college", labelpad=15)
plt.legend();
```

Another helpful visualization would be a cumulative percentage graph. To get this, for each value in the `count_students_graduated_running_total`

colum, I need to figure out what percentage that value is of `count_students_at_class_start`

. I can perform this calculation in pandas.

```
df['percent_students_graduated_of_total'] = df['count_students_graduated_running_total']/count_students_at_class_start*100
```

The visualization below shows the `percent_students_graduated_of_total`

over the years. It's clear that 4 years in, almost 60% of students have graduated.

```
df.plot(x='years_after_starting_college', y='percent_students_graduated_of_total', kind='line',
figsize=(10, 8), legend=False, style='mo-')
plt.title("Percent of Students Graduated from Starting Class", y=1.01, fontsize=20)
plt.ylabel("percent of students graduated", labelpad=15)
plt.xlabel("years after starting college", labelpad=15);
```