The compressive strength of concrete is being tested by a civil engineer. He tests 12 specimens and obtains the following data.

2236 2247 2249 2304

2235 2301 2281 2263

2318 2255 2275 2295

(a) Check the assumption that compressive strength is normally distributed. Include a graphical display in your answer.

(b) Construct a 98% two-sided confidence interval on the mean strength.

(c) Construct a 98% lower-confidence bound on the mean strength.