Mountain climbers often experience several symptoms when they reach high altitudes during their climbs. Researchers studied the effects of exposure to high altitude on human skeletal muscle tissue. They set up a 2 × 2 factorial experiment in which subjects trained for 6 weeks on a bicycle. The first factor was whether subjects trained under hypoxic conditions (corresponding to an altitude of 3,850 m) or normal conditions. The second factor was whether subjects trained at a high level of energy expenditure or at a low level (25% less than the high level). There were either 7 or 8 subjects at each combination of factor levels. The accompanying table shows the results for the response variable "percentage change in vascular endothelial growth factor mRNA."42

Prepare an interaction graph (like Figure 11.7.3).

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HypoxicNormal Low High Low High level level level level Energy Mean No, of patients7 17.7 173.295. 114.6 7 8