Which of the following is true of special cause variation?

a. It cannot be explained or understood.
b. It cannot be detected using statistical methods.
c. It accounts for about 80 to 95 percent of the observed variation in a process.
d. It occurs sporadically and can be prevented.

Awall of 0.5m thickness is to be constructed from a material which has average thermal conductivity of 1.4 w/mk. the wall is to be insulated with a material having an average thermal conductivity of 0.35 w/mk so that heat loss per square meter shall not exceed 1450 w. assume inner wall surface temperature of 1200°c and outer surface temperature of the insulation to be 15°c. calculate the thickness of insulation required.