Asample of 14 joint specimens of a particular type gave a sample mean proportional limit stress of 8.48 mpa and a sample standard deviation of .79 mpa ("characterization of bearing strength factors in pegged timber connections,"j. of structural engr., 1997: 326–332). a. calculate and interpret a 95% lower confidence bound for the true average proportional limit stress of all such joints. what, if any, assumptions did you make about the distribution of proportional limit stress? devore, jay l.. probability and statistics for engineering and the sciences (p. 302). cengage learning. kindle edition.