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# Help required in understanding technical graphs and charts

micro graphs charrts log cfu

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### #1 StevieP

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Posted 07 July 2015 - 03:57 PM

Afternoon all,

I'm currently a 3rd year on a food supply chain degree and as part of my study I am getting asked to look at a financial reporting period graph that contains finished product micro results by period. The initial data was recorded by cfu/g but the graph has been converted to Log cfu/g.

I don’t really understand what Log cfu/g means and how the author converted standard cfu/g to Log cfu/g.

Also, when reading a graph containing Log cfu/g, how can I establish what an acceptable result is and what an unacceptable result is?

I can understand standard cfu/g results but just struggling with Log cfu/g.

Can anyone help?

I look forward to your response.

Thanks.

### #2 Charles.C

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Posted 07 July 2015 - 04:42 PM

Afternoon all,

I'm currently a 3rd year on a food supply chain degree and as part of my study I am getting asked to look at a financial reporting period graph that contains finished product micro results by period. The initial data was recorded by cfu/g but the graph has been converted to Log cfu/g.

I don’t really understand what Log cfu/g means and how the author converted standard cfu/g to Log cfu/g.

Also, when reading a graph containing Log cfu/g, how can I establish what an acceptable result is and what an unacceptable result is?

I can understand standard cfu/g results but just struggling with Log cfu/g.

Can anyone help?

I look forward to your response.

Thanks.

Do you mean you don't understand how to use logarithms  or you don't understand their use in the context of cfu/g ?

Kind Regards,

Charles.C

### #3 StevieP

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Posted 07 July 2015 - 05:55 PM

Hi Charles,

A bit of both really. For example,  if I had a control of <10cfu/g for entero's but my actual sample was reported as 70cfu/g (which would be reported as out of spec). If I had lots of data that ranged fro 30cfu/g to 300cfu/g how do I work out the Log cfu/g for charting purposes?

### #4 MWidra

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Posted 07 July 2015 - 05:57 PM

MrSteveyp, in the world of science, it is sometimes necessary to convert a number to the log of it so that the graph is easier to comprehend and to make the line more linear.  This is very common.  It's called a semi-log plot for short.

https://en.wikipedia...i/Semi-log_plot

It really helps when you have an exponential relationship.

So if you have 10,000 cfu/g, on a log graph it will be labeled as "4" (the log of 104), and the next unit (1,000) will be labeled "3" (the log of 103).

Easy as pie, but not as pi, lol.

Martha

"...everything can be taken from a man but one thing:  the last of the human freedoms--to choose one's attitude in any given set of circumstances, to choose one's own way."  Viktor E. Frankl

"Life's like a movie, write your own ending."  The Muppets

### #5 MWidra

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Posted 07 July 2015 - 06:09 PM

Hi Charles,

A bit of both really. For example,  if I had a control of <10cfu/g for entero's but my actual sample was reported as 70cfu/g (which would be reported as out of spec). If I had lots of data that ranged fro 30cfu/g to 300cfu/g how do I work out the Log cfu/g for charting purposes?

You put it onto a semi-log scale.  There are graphing programs that will take your number, convert it to the log, then graph it.  Excel needs you to do the conversion.

If you use semi-log graph paper, then you put the 10,000 at a major line, then put 3,000 at the minor line that is 3 units from the 10,000.  It will not be spaced equally, it will be about 1/2 the distance between 10,000 and 1,000.

Check out this excel sheet.

semilog.xlsx   11.06KB   31 downloads

Hope that helps.

Martha

"...everything can be taken from a man but one thing:  the last of the human freedoms--to choose one's attitude in any given set of circumstances, to choose one's own way."  Viktor E. Frankl

"Life's like a movie, write your own ending."  The Muppets

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### #6 Charles.C

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Posted 07 July 2015 - 06:39 PM

Hi steve,

If you want to know about logarithms and avoid math. formulas, can try this site -

http://www.jerrydall...m/lhsp/logs.htm

But if you want to try some conversions first, can go on >>

I don’t really understand what Log cfu/g means and how the author converted standard cfu/g to Log cfu/g

As i understand you are ok with cfu/g.

Go to this on-line converter –

http://www.1728.org/logrithm.htm

make sure the top left hand box (logarithm) is highlighted

insert any value from yr graph / histogram (I guess this is what you have) in cfu/g  in the empty right hand box (just below the antilog box).

click enter tab

the value of log cfu/g will now appear at bottom

eg type in 100, > enter, click and 2 appears at bottom because log(100) = 2

alternatively click the antilog tab at top to reverse the calculation and follow reverse steps (type in 2, click enter), 100 appears in bottom.

Please revert if the above is intelligible and works because the manipulation is probably necessary to understand responses to the other half of yr post.

Hopefully the results you get will match on yr graph.

PS - it might help if you posted yr graph here but if the data is proprietary no problem

PPS - the on-line calculator is set at a default "base" of 10, don't change this button since log values depend on the choice of the base. i assume all your data for log cfu/g is also using "base 10".

Kind Regards,

Charles.C

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### #7 StevieP

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Posted 07 July 2015 - 07:06 PM

thanks Martha

### #8 StevieP

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Posted 07 July 2015 - 07:07 PM

thanks Charles, I'll have a go a this stuff.... big help!

### #9 CMHeywood

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Posted 08 July 2015 - 02:18 PM

If the range of numbers is very large (such as 100 to 1,000,000) the small numbers can get "hidden" in a regular graph.  Changing the numbers to the logarithm makes it easier to see small numbers and large numbers on the same graph.

Log = Logarithm.   1000 = 10 x 10 x 10 = 103.   Log(1000) = Log(103) = 3.

You can use Excel formula to do this:  Log10(value).   Log10(1000) = 3

You can use Excel formula to convert Log back to regular number:  10^value.   10^3 = 1000

Log10(C3) or 10^C3 if value is in cell C3.

If 10,000 (104)is the maximum limit, then Log(10000) = 4 is the maximum limit.

Shift 6 = ^         In Excel, LOG(1000) = LOG10(1000).

LOG10 uses base 10 only.   You can use other numbers for base if using LOG() formula.

LOG(8,2) = Log(8)base 2 = Log(23)base 2 = 3 (base 2).  8 = 2 x 2 x 2 = 23

10

3

103.

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