I am doing an experiment using tomato seedlings. The plants used as starting material for the experiment are chosen as uniformly as possible from a larger population of seedlings. As the standard protocol, I selected 30 similar seedlings as the one I used for the experiment, then measured some parameters (fresh weight, leaf area, shoot & root length, etc.) and presented them as mean ± SD to represent the starting material.
To my disappointment, despite the plants being chosen carefully, the SD were still quite large, up to 13% - 17% of the mean in some parameters. I am not sure if this is acceptable. So I want to know how small the SD of starting material should be in a biological experiment. Is there any standard for it? Does it depend on the number of plants I use for my experiment?
I am not Professional in biostatistic but this is what i learn form the biostatistic classes i attend and i apply in my research:
first you should know if your data is parametric or not? by dividing Mean/SE= if the number you get out side this range ( -1.96, +1.96) this mean your data is not normally distrbuted and you should analyze you data non-parametrically and express your result as Median +_ IQR to solve your problem.
if you use Graphpad to analyze your data he will directly show you the way but if you use SPSS program you should do the step above. but be careful always parametric test are more powerful than non-parametric test.
Dear Duy, for those big enough populations their characters tend to have a normal distribution. So, probably you're using a lower number of plants. A preselection regarding, for example, the length of plants don't assure that all they growth with the same extention. Individual growth is determined for many, many internal and external factors that you can't control all of them. In this cases, the use of an experimental design could also help you to reduce the effect of error caused by these uncontrolled factors. Good luck
As James Berreen said, it would be good to provide more information on your experimental design and aims for the community to better answer your question.
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