<commit_before>### Changelogs

#### 0.7.1 
- addition of a l2 `penalizer` to `CoxPHFitter`.
- dropped Fortran implementation of efficient Python version. Lifelines is pure python once again!
- addition of `strata` keyword argument to `CoxPHFitter` to allow for stratification of a single or set of 
categorical variables in your dataset.
- `datetimes_to_durations` now accepts a list as `na_values`, so multiple values can be checked. 
- fixed a bug in `datetimes_to_durations` where `fill_date` was not properly being applied.
- Changed warning in `datetimes_to_durations` to be correct. 
- refactor each fitter into it's own submodule. For now, the tests are still in the same file. This will also *not* break the API. 


#### 0.7.0
- allow for multiple fitters to be passed into `k_fold_cross_validation`. 
- statistical tests in `lifelines.statistics`. now return a `StatisticalResult` object with properties like `p_value`, `test_results`, and `summary`.  <commit_msg>update changelog<commit_after>### Changelogs

#### 0.8.0
 - reorganized lifelines directories: 
    - moved test files out of main directory. 
    - moved `utils.py` into it's down directory.
    - moved all estimators `fitters` directory.
 - added a `at_risk` column to the output of `group_survival_table_from_events` and `survival_table_from_events`
 - added sample size and power calculations for statistical tests. See `lifeline.statistics. sample_size_necessary_under_cph` and `lifelines.statistics. power_under_cph`. 
 - fixed a bug when using KaplanMeierFitter for left-censored data. 


#### 0.7.1 
- addition of a l2 `penalizer` to `CoxPHFitter`.
- dropped Fortran implementation of efficient Python version. Lifelines is pure python once again!
- addition of `strata` keyword argument to `CoxPHFitter` to allow for stratification of a single or set of 
categorical variables in your dataset.
- `datetimes_to_durations` now accepts a list as `na_values`, so multiple values can be checked. 
- fixed a bug in `datetimes_to_durations` where `fill_date` was not properly being applied.
- Changed warning in `datetimes_to_durations` to be correct. 
- refactor each fitter into it's own submodule. For now, the tests are still in the same file. This will also *not* break the API. 


#### 0.7.0
- allow for multiple fitters to be passed into `k_fold_cross_validation`. 
- statistical tests in `lifelines.statistics`. now return a `StatisticalResult` object with properties like `p_value`, `test_results`, and `summary`.  
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