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Help deciphering a figure in an article...

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Help deciphering a figure in an article...

Postby Melissah » Mon Dec 03, 2007 7:45 pm

Hello anyone who can help me :)

I'm reading an article which I am trying to sum up to present to some other students in my program, and am having a problem with one of the figures... it is showing CFU/mL (log) vs. time, and showing that higher concentrations of gallium lower the number of colony forming units, which I get. However... what exactly is it measured in if they say "CFU/mL(log)"? I don't quite get the "log" part. To me, it doesn't appear that the figure is showing much... it decreases from ~9 CFU/mL to ~6 CFU/mL... Is this actually a significant decrease (since they provide no stats!), or what does the "log" distinction have to do with it??

Here is a link to the paper I'm reading: http: // www. pubmedcentral.nih.gov/picrender.fcgi?artid=1810576&blobtype=pdf

On the second page, figure 1...

Anyone with any insights would be more than welcome!!!

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Postby blcr11 » Mon Dec 03, 2007 8:40 pm

CFU/ml stands for colony forming units per ml and is the number of viable cells per ml of culture. The (log) designation is just to remind the reader that they've plotted the data on a logarithmic scale--not uncommon for these types of data. They did do statistics. It's not done on the data in figure 1A directly, but look at figure 1B.
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Postby Melissah » Mon Dec 03, 2007 8:50 pm

Ok, the logarithmic scale is where I'm getting confused. I do understand the concept of CFU, but on the log scale, is it still correct to state that the CFU/mL are 8 or 6 or what have you, or does this scale change that in some way?
Further to that... since they don't really state the stats they did (but I do see the error bars and p value for figure 1b), is a reduction from 8 to 6 CFU/mL actually a significant amount? To me it doesn't seem like much, though I suppose that it could depend on personal opinion...?
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Postby blcr11 » Mon Dec 03, 2007 8:54 pm

That's a difference of two orders of magnitude you're talking about. Take the antilog of 8 and compare it to the antilog of 6 and see what you get.
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Postby Melissah » Mon Dec 03, 2007 8:59 pm

Ooooh yes, that's the detail I was looking for!
(it's been awhile since I've done anything with logarithms, haha)

Thank you VERY much for clearing that one up!!!

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Postby blcr11 » Mon Dec 03, 2007 9:01 pm

You're welcome. Good luck with the presentation.
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