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Reading values from a logarithmic scaled x-axis

  1. #1
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    Reading values from a logarithmic scaled x-axis

    Hey, I'm having trouble reading off values from a couple of charts for a molecular biology lab. I'm supposed to get the X value for each of the four lines at 50% of the max. response (i.e. where a line drawn from 50 on the y axis would intercept the lines I've plotted). Log scales are hard to work with though, is there any way I can get excel to do this for me? I've attached a screen grab of them to show what I'm talking about. Thanks to anyone who can help.
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  2. #2
    Forum Expert mrice's Avatar
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    Why not use the log(10) of the numbers on the X-axis on your dose response plot which will be linear.

    Are you using a parameterised formula to define your sigmodal function or just letting Excel generate a best fit? If its the former, you could obviously insert 50% as the answer and work backwards.
    Martin

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  3. #3
    Forum Guru shg's Avatar
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    You could add major and minor gridlines to the x axis, which would make it easier to read and interpolate visually, or interpolate it mathematically.

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