I need some help with statistics and excel.
Basically, I have to plot a graph Absorbance (at 550 nm) vs time (hours) for E.coli. I measured absorbance value every hour for 4 hours. So I have 4 values for every hour, then I'm calculating the mean (for every single hour of course) and plot a graph. And then my assignment says I have to do error bars and sadly statistics is my weakness. I do not understand why do I need them and I do not know how to obtain them- I mean there's an option 'Y error bars' when I click on the graph and then I can choose 'error category' such as constant value, percentage, standard error, standard deviation etc, and also 'error indicator' like positive and negative, positive, negative. My guess is: standard error as 'error category' and positive and negative in 'error indicator' part.
Could anyone tell me how should I do it (if there's any formula or anything like that)? and also shed some light on why do I need them? I really have no clue what they represent and if they have any importance.
Thanks in advance