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A study examined the survival of patients with heart disease and pet ownership. Each patient with heart disease was classified as having a pet or not and whether they survived for one year.
The data are given below.
| Pet | Ownership | |
| Patient status | No | Yes |
| Alive | [x] | 50 |
| Dead | 10 | 5 |
In carrying out a chi-square test assuming no relationship between pet ownership and survival of patients, how many degrees of freedom are there?