Once we take average of control readings as 100%, is it incorrect to back calculate percentage of replicate readings of control to get standard deviation for control? Is it necessary to drop standard deviation in control as it is taken as 100% arbitrary?
Well, standard deviation must be taken since when you report it in a paper, you will have to show the error bar. But when you are calculating % viability, you must consider the average and not the single reading, because it will give you three % viability readings. So, how will you decide which one to report? So take s.d for the bar graph and average to report % viability.
I take average of control readings as 100% and calculate the percent of control for rest of the readings including the original triplicate readings of control. Formula; (test readingX100)/average of readings of control.
Using calculated triplicate control in percentage, I calculate SD for control.
Is it a wrong method?
See this one: i got readings for controls 1,1,1. Avg= 1. I got test readings, 0.5,0.6,0.4. Avg= 0.5. For % viability i'll do this: 0.5/1*100 ( in your case, its correct) = 50%. So your test gives a 50% viable family of cells. For S.D. you will select (1,1,1- which are the original readings of control) and calculate S.D. I hope its helpful.
Its very rare that we get 1,1,1 as controls. We might get e.g. 0.99, 0.97, 0.96. We take average of these as 100%. And for standard deviation we have to take the same unit which we are using to report. If we are using average of readings, we have to use actual readings for SD. But if we are reporting in percentage, we need to convert actual readings to percentages and then take SD from triplicates. Its totally wrong if you convert average into percentage and take SD of actual readings.
It was an example. Moreover it was an example of one time triplicate reading in which case the above calculation is correct. But, if you are talking about 3 to 4 times repeated experiment with triplicate readings then you will have to convert them to % and then take the SD.
How do we calculate the concentration of the extract to be used for future assay from using MTT assay, if our extract is cyto-protective in nature. Because I am getting more OD of test sample than control in all the concentrations of extracts. So I am getting % viability always higher than 100%. How to choose the concentration of extract for future experiments. Please suggest.
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