4.35 Mean versus median for age at first marriage: The mean age at first marriage was 31.1 for men and 29.1 for women in Canada in 2008 (http: /..r@-eng.jsp?iid=78). The median age at first marriage was 28.9 for men and 26.9 for women in the United States in 2011

(http://factfinder.census.gov/faces/tableservices/jsf/pages/productview.xhtml?pid=ACS_11_1YR_B12007&prodType=table). Beyond the fact that these data are from slightly different years, explain why we cannot directly compare these measures of central tendency to make cross-national comparisons.