A study looked at the relationship between condom use and the spread of AIDS, by following heterosexual couples in which one partner was infected with HIV. The couples were classified by their condom use. Of the 181 couples who always used condoms, 4 partners became infected with HIV. Of the 59 couples who did not always use condoms, 9 partners became infected. Create a 95% confidence interval for the difference between proportions of partners who became infected with HIV.