Estimate the coefficients of a filtered monotonic polynomial IRT model.

1 |

`data` |
N(subjects)-by-p(items) matrix of 0/1 item response data. |

`thetaInit` |
Initial theta ( |

`item` |
Item number for coefficient estimation. |

`startvals` |
Start values for function minimization. Start values are in the gamma metric (see Liang & Browne, 2015) |

`k` |
Order of monotonic polynomial = 2k+1 (see Liang & Browne, 2015). k can equal 0, 1, 2, or 3. |

`eps` |
Step size for gradient approximation, default = 1e-6. If a convergence failure occurs during function optimization reducing the value of eps will often produce a converged solution. |

As described by Liang and Browne (2015), the filtered polynomial model (FMP) is a quasi-parametric
IRT model in which the IRF is a composition of a logistic function and a polynomial function, *m(θ)*, of degree 2k + 1.
When k = 0, *m(θ) = b_0 + b_1 θ*
(the slope intercept form of the 2PL). When k = 1, 2k + 1 equals 3 resulting in
*m(θ) = b_0 + b_1 θ + b_2 θ^2 + b_3 θ^3*. Acceptable values of k = 0,1,2,3. According to
Liang and Browne, the "FMP IRF may be used to approximate any IRF with a continuous derivative arbitrarily
closely by increasing the number of parameters in the monotonic polynomial" (2015, p. 2) The FMP model assumes that the
IRF is monotonically increasing, bounded by 0 and 1, and everywhere differentiable with respect to theta (the latent trait).

`b` |
Vector of polynomial coefficients. |

`gamma` |
Polynomial coefficients in gamma metric (see Liang & Browne, 2015). |

`FHAT` |
Function value at convergence. |

`counts` |
Number of function evaluations during minimization (see optim documentation for further details). |

`AIC` |
Pseudo scaled Akaike Information Criterion (AIC). Candidate models that produce the smallest AIC suggest the optimal number of parameters given the sample size. Scaling is accomplished by dividing the non-scaled AIC by sample size. |

`BIC` |
Pseudo scaled Bayesian Information Criterion (BIC). Candidate models that produce the smallest BIC suggest the optimal number of parameters given the sample size. Scaling is accomplished by dividing the non-scaled BIC by sample size. |

`convergence` |
Convergence = 0 indicates that the optimization algorithm converged; convergence=1 indicates that the optimization failed to converge. |

Niels Waller

Liang, L. & Browne, M. W. (2015). A quasi-parametric method for fitting flexible item response functions.
*Journal of Educational and Behavioral Statistics, 40*, 5–34.

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## Not run:
## In this example we will generate 2000 item response vectors
## for a k = 1 order filtered polynomial model and then recover
## the estimated item parameters with the FMP function.
k <- 1 # order of polynomial
NSubjects <- 2000
## generate a sample of 2000 item response vectors
## for a k = 1 FMP model using the following
## coefficients
b <- matrix(c(
#b0 b1 b2 b3 b4 b5 b6 b7 k
1.675, 1.974, -0.068, 0.053, 0, 0, 0, 0, 1,
1.550, 1.805, -0.230, 0.032, 0, 0, 0, 0, 1,
1.282, 1.063, -0.103, 0.003, 0, 0, 0, 0, 1,
0.704, 1.376, -0.107, 0.040, 0, 0, 0, 0, 1,
1.417, 1.413, 0.021, 0.000, 0, 0, 0, 0, 1,
-0.008, 1.349, -0.195, 0.144, 0, 0, 0, 0, 1,
0.512, 1.538, -0.089, 0.082, 0, 0, 0, 0, 1,
0.122, 0.601, -0.082, 0.119, 0, 0, 0, 0, 1,
1.801, 1.211, 0.015, 0.000, 0, 0, 0, 0, 1,
-0.207, 1.191, 0.066, 0.033, 0, 0, 0, 0, 1,
-0.215, 1.291, -0.087, 0.029, 0, 0, 0, 0, 1,
0.259, 0.875, 0.177, 0.072, 0, 0, 0, 0, 1,
-0.423, 0.942, 0.064, 0.094, 0, 0, 0, 0, 1,
0.113, 0.795, 0.124, 0.110, 0, 0, 0, 0, 1,
1.030, 1.525, 0.200, 0.076, 0, 0, 0, 0, 1,
0.140, 1.209, 0.082, 0.148, 0, 0, 0, 0, 1,
0.429, 1.480, -0.008, 0.061, 0, 0, 0, 0, 1,
0.089, 0.785, -0.065, 0.018, 0, 0, 0, 0, 1,
-0.516, 1.013, 0.016, 0.023, 0, 0, 0, 0, 1,
0.143, 1.315, -0.011, 0.136, 0, 0, 0, 0, 1,
0.347, 0.733, -0.121, 0.041, 0, 0, 0, 0, 1,
-0.074, 0.869, 0.013, 0.026, 0, 0, 0, 0, 1,
0.630, 1.484, -0.001, 0.000, 0, 0, 0, 0, 1),
nrow=23, ncol=9, byrow=TRUE)
ex1.data<-genFMPData(NSubj = NSubjects, bParams = b, seed = 345)$data
## number of items in the data matrix
NItems <- ncol(ex1.data)
# compute (initial) surrogate theta values from
# the normed left singular vector of the centered
# data matrix
thetaInit <- svdNorm(ex1.data)
## earlier we defined k = 1
if(k == 0) {
startVals <- c(1.5, 1.5)
bmat <- matrix(0, NItems, 6)
colnames(bmat) <- c(paste("b", 0:1, sep = ""),"FHAT", "AIC", "BIC", "convergence")
}
if(k == 1) {
startVals <- c(1.5, 1.5, .10, .10)
bmat <- matrix(0, NItems, 8)
colnames(bmat) <- c(paste("b", 0:3, sep = ""),"FHAT", "AIC", "BIC", "convergence")
}
if(k == 2) {
startVals <- c(1.5, 1.5, .10, .10, .10, .10)
bmat <- matrix(0, NItems, 10)
colnames(bmat) <- c(paste("b", 0:5, sep = ""),"FHAT", "AIC", "BIC", "convergence")
}
if(k == 3) {
startVals <- c(1.5, 1.5, .10, .10, .10, .10, .10, .10)
bmat <- matrix(0, NItems, 12)
colnames(bmat) <- c(paste("b", 0:7, sep = ""),"FHAT", "AIC", "BIC", "convergence")
}
# estimate item parameters and fit statistics
for(i in 1:NItems){
out <- FMP(data = ex1.data, thetaInit, item = i, startvals = startVals, k = k)
Nb <- length(out$b)
bmat[i,1:Nb] <- out$b
bmat[i,Nb+1] <- out$FHAT
bmat[i,Nb+2] <- out$AIC
bmat[i,Nb+3] <- out$BIC
bmat[i,Nb+4] <- out$convergence
}
# print output
print(bmat)
## End(Not run)
``` |

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