Cyntoia Brown was sentenced to life in prison in 2004 for a man she killed when she was 16 years old.

This week though, Brown was granted clemency by the Tennessee governor after appeals by her lawyers claiming that she was a victim of sex trafficking who feared for her life. Brown, now aged 30, will remain on parole supervision for 10 years so long as she retains a job and participates in regular counseling sessions.

The man Brown killed was Johnny Allen, who had taken her to his house that night and paid her for sex. Though he wasn’t her partner, the case prompted me to look at the gap between sentencing of men and women who kill intimate partners in the US.

Statistics cited by the ACLU and the Women’s March suggest a wide gender gap in sentencing. The average prison sentence for men who kill their female partners is two to six years (the illustration here takes the midpoint of those values). By contrast women, who kill their partners are sentenced on average to 15 years.

Despite its widespread use, the statistic is dated. It was first published by the National Coalition Against Domestic Violence in 1989. It remains true that most women who kill their partners cite self-defense as a motive. In fact, 70-80% of incarcerated women report intimate partner violence. But there doesn’t appear to be any recent analysis of sentencing to see if this gender gap in sentencing remains the same.

Since the statistic was first published, one of the clearest changes in the US prison system has been the dramatic increase in women’s incarceration rates. Research also suggests that women are given harsher punishments when they have committed crimes that are perceived as more masculine, such as murder.

More recent statistics from other countries suggest that intimate partner violence committed by men continues to be treated with leniency. In Ireland, men who are convicted of the manslaughter of current or former partners serve an average of 2.8 years less time in jail than other men convicted of the same charge against people who were not their partners.

This is a column that illustrates numbers from the news each week. Have feedback or ideas for future columns? Write to me: mona.chalabi@theguardian.com