The IPUMS-CPS is a large random sample of U.S. households that measures a variety of data. One study using this survey found that the average number of hours workedper week for mothers in cohabiting unions with other women was 30.3. Source: Brown, S. L., Manning, W. D., Payne, K. K. (2016). Family structure and children's economic well-being: Incorporating same-sex cohabiting mother families. Population Research and Policy Review, 35, 1-21.a. Give notation for the parameter of interest and define the parameter in this context