Vitamins in bread. Does bread lose its vitamins when stored? Here are data on the vitamin C content (milligrams per 100 grams of flour) in bread baked from the same recipe and stored for 1, 3, 5, or 7 days. The 10 observations are from 10 different loaves of bread.
The loss of vitamin C over time is clear, but with only 2 loaves of bread for each storage time we wonder if the differences among the groups are significant.
(a) Use the Kruskal-Wallis test to assess significance and then write a brief summary of what the data show.
(b) Because there are only 2 observations per group, we suspect that the common chi-square approximation to the distribution of the Kruskal-Wallis statistic may not be accurate. The exact P-value (from the SAS software) is P = 0.0011. Compare this with your P-value from part (a). Is the difference large enough to affect your conclusion?