Design of direct binding and sandwich elisa | Quick Homework Help

I have to write a lab report about ELISA (Direct and Sandwich). One of the tasks is to draw histograms from the data we obtained.Basically we have three serums and we are looking for IgG and IgE. Each sample has two wells so two data is available.
After finding the average of G1, E1, G2, E2, G3 and E3 plotting thehistogram is not a problem. But the next question is asking us todraw error bars!!! I have gone through lots of articles andjournals and everyone uses error bars in their ELISA datapresentation but no one describes how you would do one?

Any ideas about how to place error bars into histograms? Do I haveto find the Standard Deviation? Student’s t Test? Percentage errorbar?

Another question: Explain the rationale of the design of direct binding and sandwich ELISA. And advantages of using a peptidemarker instead of a full length antigen in immunodiagnosis.