Hi,

I am using the following dataset and fitting a 4 parameter logistical curve using drm. However, there is no p value returned for any of the fitted coefficients and a warning message of NaNs produced is returned. I tried using both LL.4 and LL2.4. What does the warning message imply? The data fitted here does not quite match up with the results of Masterplex software. What may I be doing wrong? Please help.

Weights <-1/(y1^2)
list(all)
[[1]]
        x1     y1
1  1500.00 439908
2  1500.00 464838
3  1500.00 467593
4   150.00  26126
5   150.00  26639
6   150.00  26524
7    25.00   3482
8    25.00   3555
9    25.00   3438
10    5.00    790
11    5.00    826
12    5.00    805
13    2.00    443
14    2.00    464
15    2.00    469
16    1.00    337
17    1.00    355
18    1.00    364
19    0.50    278
20    0.50    279
21    0.50    307
22    0.25    241
23    0.25    266
24    0.25    276

FourPL.fit <- drm(y1~x1,weights=weights,data=all,fct=LL2.4())

summary(FourPL.fit <- drm(y1~x1,weights=weights,data=all,fct=LL2.4()))

Model fitted: Log-logistic (log(ED50) as parameter) (4 parms)

Parameter estimates:

                   Estimate    Std. Error       t-value p-value
b:(Intercept)     -1.144871      0.068434    -16.729579       0
c:(Intercept)    241.482714      7.112223     33.953199       0
d:(Intercept) 423431.818035            NA            NA      NA
e:(Intercept)      7.319215            NA            NA      NA

Residual standard error:

 0.0001265872 (20 degrees of freedom)
Warning message:
In sqrt(diag(varMat)) : NaNs produced


> summary(FourPL.fit.1 <- drm(y1~x1,weights=weights,data=all,fct=LL.4()))

Model fitted: Log-logistic (ED50 as parameter) (4 parms)

Parameter estimates:

                  Estimate   Std. Error      t-value p-value
b:(Intercept)     -1.20654      0.07606    -15.86309  0.0000
c:(Intercept)   -226.35200    128.30261     -1.76420  0.0930
d:(Intercept) 468060.34801           NA           NA      NA
e:(Intercept)    292.10119    132.92109      2.19755  0.0399

Residual standard error:

 0.003207409 (20 degrees of freedom)
Warning message:
In sqrt(diag(varMat)) : NaNs produced



Master Plex summary:
Initial Top     462020.7967
Initial Bottom  234.9
Initial MidPoint        784.0181741
Initial HillSlope       1
Weighting       1/y^2
Max Iteration   500


Thanks,
Abhi



	[[alternative HTML version deleted]]

