```python
import pandas as pd
import matplotlib.pyplot as plt
def donut(values, labels, title):
fig, ax = plt.subplots(figsize=(2.4,2.4))
wedges, _ = ax.pie(values, wedgeprops={'width': 0.4})
ax.set_title(title)
plt.legend(labels,bbox_to_anchor=(0, 1), loc='upper right', ncol=1)
plt.show()
df = pd.DataFrame({
'systematic reviews': 3,
'technical notes': 4,
'case studies': 5,
'case-control studies': 1,
'letter to the editor': 1
})
donut(df['k'], df['study_design'], 'Types of Studies Published')
```