im_1 wrote  on Sun, 27 Nov 2005 10:55:03 -0800:

i> can someone tell me asap how i can calculate half lives for
i> a curve in microsoft excel?

More explanation please? I know about half lives of isotopes and
reactions but not of curves.

James Silverton.

Hi,

Do you mean by 'half-life of a CURVE', the x-value when the y-value is 50%
of the initial y-value (i.e., y-value when x is zero)?  Also, I assume that
your x-axis corresponds to time.

If the curve corresponds to an exponential decay of something, i.e., the
equation for the curve is "y= a*exp(-b*x)" where y is the quantity of that
thing and x is time, and if you know the values of the parameters ('a' and
'b'), the half-life ican be calculated using the following equation,

half-life = ln(2)/b = 0.6931/b

Note: For an exponential-decay as the above, a plot of ln(y) vs x would be
linear with a slope of -b and a y-intercept of ln(a).

If the decay is non-exponential the formula is different. If the functional
form is known (maybe from the trendline equation of an appropriate type), you
can calculate the half-life either analytically using an appropriate formula
or find it using the "Solver" in Excel.  Please post details about the
functional form of the curve.

Regards,
B. R. Ramachandan