Betty Clarke is a 28 year old woman with a

Elevated serum amylase levels have been frequently observed in patients with bulimia nervosa and may be a marker of symptom severity. A study attempted to determine whether elevations in amylase had a stronger relationship with large binge eating episodes or with purging behaviors. They utilized a sample recruited from local advertisements. Participants included 26 women with bulimia nervosa (binging and purging); 14 women with “purging disorder,” a subtype of eating disorder not otherwise specified; and 32 women without any eating disorders. The frequency of self-induced vomiting in the clinical samples was similar. Significant elevations in serum amylase levels were identified in the bulimia nervosa group (60.7 ± 25.4 IU/liter, mean ± sd) in comparison to the “purging disorder” group (44.7 ± 17.1 IU/L, p < .02) and to the control group (49.3 ± 15.8, p < .05). No significant differences were found between the "purging disorder" group and the control group. Which of the following poses the most important threat to the internal validity of this study? Convenience sample Experimental design Lack of anorexia nervosa group Small sample size

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