- iii) Children aged 17 and younger engaged in activities that are designated as “ _hazardous”_ (affecting the child’s safety, physical and mental development) or the “ _worst forms of child labor”_ (e.g. children in bondage or forced labor, commercial sexual exploitation, illicit activities and armed conflict, among others).

Based on these definitions, ILO (2013) estimates that there were 264 million children ages 5 to 17 engaged in an economic activity in the world (16.7 percent), of which 168 million children were involved in child labor. About 51 percent of child laborers (85.3 million) were engaged in hazardous work.

Academic researchers, however, rarely measure child labor following the ICLS resolution outlined above. The child labor literature often uses the terms “ _child labor”_ and “ _children engaged in productive activities”_ interchangeably. Moreover, there is considerable heterogeneity in the types of productive activities considered in this literature: some papers define children in wage work as child laborers while others include children engaged in any economic activity for the market (paid or unpaid) or even household chores. The challenge of using wage work as an indicator for child labor is that many children working outside the household are not paid. However, not all forms of unpaid activities are considered detrimental to children’s development. For that reason, some studies apply an hour limit to the number of domestic hours a child works to consider activities where children regularly participate (Edmonds 2008).

There is also variation in the reference period applied in these studies. Some studies look at work in the 7 days prior to the household survey while other studies focus on work in the past one or twelve months. This heterogeneity complicates the comparison of program effects across studies. We report the definition and reference time used in each paper (when available) in our review; we also use child labor and children engaged in an economic activity interchangeably throughout the paper.

It is important to note that there could be unexplained inconsistency in child labor statistics even when a single definition of child labor is used. Guarcello et al. (2010), for instance, documents large discrepancies in child labor statistics between independent national surveys within the same country that ranges from 20 to 30 percentage points, even after accounting for differences in sample design. For instance, in Cameroon, a comparison between the Multiple Indicator Cluster Survey (MICS 2000) and a Priority Survey (2001) shows a decline in child labor from 64 percent in the MICS survey to 16 percent in the Priority Survey one year later. In Senegal, the Demographic Health Survey (DHS 2005) reports 35.2 percent of children as engaged in an economic activity while the Statistical Information and Monitoring Programme on Child Labour (SIMPOC 2005) survey of the same year reports 22.3 percent of children as working.

Despite the increasing sources of information on child labor over the past decade, there is not much evidence on the validity of data collection methods (Edmonds 2008). Child labor could be affected by measurement error due to several factors, for example, the survey information is collected primarily using standard household surveys that target adult work, i.e., formal jobs rather than unpaid and family work/enterprise jobs. Likewise, due to budgetary constraints 5