Dear Haixin,

Thank you for your interest in my work. The letters indicate significance groups. Suppose three bars with letters a, ab and b. The first two bars share the same letter (a), so they are not significantly different. The same is true for the last two bars (letter b). However, the first and last bar don't share the same letter, so they differ significantly. It's a clean way to visualize the results of multiple pairwise statistical comparisons (t-tests) without making a mess of arrows and lines on the plot. The Roman letters belong to the bars of CODtot, the Greek letters belong to the bars of CODdiss.

Dear Dr. Francis,

Wow, it's surprising that you can reply me so quickly and I am so happy now. I understand your answers and interpretations. Here is my anothor question: can these letters be obtained from t values or p values? For example, when p < 0.05, which letter should be choosed? (Is my question right?)

Thank you very much.

P.S. I am reading your other articles including the dissertation required for the degree of PhD., so interesting! :)

Yes, if the p value is < 0.05, the bars have a different letter, if the p value is > 0.05, they share the same letter

Best,

Francis

Nice. And if one letter is a, and the other is ab, is the p value of them between 0.05 and 0.1?