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 0.79 mpa ("characterization of bearing strength factors in pegged timber connections," j. of structural engr., 1997: 326332). 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?