A sample 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). calculate 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?