When you use the very simple structure method, you can choose between the VSS and the MAP criteria, that not ever reach to identical conclusions. Of course your VSS complexity 2 value is greater than the value of VSS complexity 1: you have to consider the greater value, so you can accept the complexity 2 solution (if I'm not mistaking about your data). However, check also the Velicer MAP criterion, because the VSS criterion not ever is optimal. How many factors the MAP suggests you to retain? Are MAP and VSS in agree?
I suggest you to perform also a parallel analysis, submitting the tetrachoric correlation matrix (since your data are dichotomic).
A last suggestion. Recently, Ruscio and Roche have proposed a novel method to determine the optimal factor solution by comparison data (CD). Rather than generating random data, like parallel analysis does, this method use a simulation technique that reproduces the research data varying the factor structure, so it compares the correlation matrix of your data with that from simulated 1 factor data, 2 factors, 3 factors... and so on (you choose how many factors the program has to generate and compares). Although this is a recent method, I think it is promising, and so I suggets you to try it. The authors provide the R code to implement this method here. You have only to downlod the page as a text file and then import in R the program (source(txt file directory)).
Once you have imported the script, the command to conduct the analysis is very simple. In this case, x is a data frame with 9 variables, created such that there are three correlated factors:
> EFA.Comp.Data(Data=x, F.Max=9, Graph=T) # use the method on the data frame 'x'
# extract from 1 to 9 factors
# and plot the results
Number of factors to retain: 3
The program output returns you the number of factors to retain and in the graph you can see the number of factors and the relative RMSR eigenvalue. Note that when your data frame has many variables, the simulation process is slow, so you have to wait a bit for the results.
If I perform a VSS on the same data, the results are:
> vss(x)
Very Simple Structure
Call: VSS(x = x, n = n, rotate = rotate, diagonal = diagonal, fm = fm,
n.obs = n.obs, plot = plot, title = title)
VSS complexity 1 achieves a maximimum of 0.79 with 1 factors
VSS complexity 2 achieves a maximimum of 0.88 with 6 factors
The Velicer MAP criterion achieves a minimum of 0.06 with 3 factors
Velicer MAP
[1] 0.10 0.10 0.06 0.10 0.16 0.27 0.50 1.00
Very Simple Structure Complexity 1
[1] 0.79 0.68 0.68 0.68 0.68 0.68 0.68 0.69
Very Simple Structure Complexity 2
[1] 0.00 0.87 0.87 0.86 0.86 0.88 0.88 0.75
As you can see, the MAP criterion is the only that give the correct 3 factors solution. And if I do a parallel analysis:
> fa.parallel(x)
Parallel analysis suggests that the number of factors = 3 and the number of components = 3
The PA results are correct. This is the reason why I suggest you to use both PA and CD, and compare the two techniques. And if you want to use the VSS method, the MAP criterion is more reliable.
Hope this helps. Cheers.
Andrea
Reference:
Ruscio, J., & Roche, B. (2012). Determining the number of factors to retain in an exploratory factor analysis using comparison data of known factorial structure. Psychological Assessment, 24, 282-292. (PubMed abstract)
