Number of events gives cell number, but not volume. So you know how many cells were processed, but not the concentration. If you want to use cell counting beads (which are at a known concentration), you can use this to establish a relationship between concentration of the standard beads and concentration of your unknown sample. 

Number of events gives cell number, but not volume. So you know how many cells were processed, but not the concentration. If you want to use cell counting beads (which are at a known concentration), you can use this to establish a relationship between concentration of the standard beads and concentration of your unknown sample. 

To add on to what David said, with most flow cytometers, you cannot control precisely how much sample you acquire. Even if you know your sample is 200 ul, you cannot suck the whole thing up or you will get air bubbles in your cytometer. Therefore, you add (for example) 100 ul of cell counting beads to 100 ul of your sample and acquire that. You analyze the results to find out the ratio of cells of interest to beads. Multiply that by the known concentration of cell counting beads to get the concentration of your cells of interest.

There are also a few flow cytometers, such as the BD Accuri and MACSQuant, that are capable of sucking up whatever volume of sample you tell it to. You don't need cell counting beads with these machines, as they can give you the events/ul.

Hello

I agree with researchers , given you good answers , for more information see links here

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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2822558/

http://www.flow-cytometry.us/

Thank you very much for your comments

As with CFUs in microbiology we are acknowledging that an "event" is likely one cell passing by the detectors but there is also the possibility that a clump of cells  or an agglomerate of debris is being registered as one cell.  

Yes, number of cells passing the laser