# Comparison of Correlation Method 1 and Correlation Method 2

#### 2018-06-28

There are two simulation pathways which differ primarily according to the calculation of the intermediate correlation matrix Sigma. Note, unless otherwise indicated, the functions referenced below come from SimMultiCorrData.

## Methods Used in Both Pathways:

First, the intermediate correlation calculations which are equivalent in the two pathways will be discussed by variable type.

### Ordinal Variables:

Correlations are computed pairwise. If both variables are binary, the method of Demirtas et al. (2012) is used to find the tetrachoric correlation (code adapted from BinNonNor::Tetra.Corr.BB). The tetrachoric correlation is an estimate of the binary correlation measured on a continuous scale. The assumptions are that the binary variables arise from latent normal variables, and the actual trait is continuous and not discrete. This method is based on Emrich and Piedmonte (1991)’s work, in which the joint binary distribution is determined from the third and higher moments of a multivariate normal distribution:

Let $$\Large Y_{1}$$ and $$\Large Y_{2}$$ be binary variables with $$\Large E[Y_{1}] = Pr(Y_{1} = 1) = p_{1}$$, $$\Large E[Y_{2}] = Pr(Y_{2} = 1) = p_{2}$$, and correlation $$\Large \rho_{y1y2}$$.

Let $$\Large \Phi[x_{1}, x_{2}, \rho_{x1x2}]$$ be the standard bivariate normal cumulative distribution function, given by: $\Large \Phi[x_{1}, x_{2}, \rho_{x1x2}] = \int_{-\infty}^{x_{1}} \int_{-\infty}^{x_{2}} f(z_{1}, z_{2}, \rho_{x1x2})\ dz_{1} dz_{2},$ where $\Large f(z_{1}, z_{2}, \rho_{x1x2}) = [2\pi\sqrt{1 - \rho_{x1x2}^2}]^{-1} * exp[-0.5(z_{1}^2 - 2\rho_{x1x2}z_{1}z_{2} + z_{2}^2)/(1 - \rho_{x1x2}^2)].$ Then solving the equation $\Large \Phi[z(p_{1}), z(p_{2}), \rho_{x1x2}] = \rho_{y1y2}\sqrt{p_{1}(1 - p_{1})p_{2}(1 - p_{2})} + p_{1}p_{2}$ for $$\Large \rho_{x1x2}$$ gives the intermediate correlation of the standard normal variables needed to generate binary variables with correlation $$\Large \rho_{y1y2}$$. Here $$\Large z(p)$$ indicates the $$\Large p^{th}$$ quantile of the standard normal distribution.

To generate the binary variables from the standard normal variables, set $$\Large Y_{1} = 1$$ if $$\Large Z_{1} \le z(p_{1})$$ and $$\Large Y_{1} = 0$$ otherwise. Similarly, set $$\Large Y_{2} = 1$$ if $$\Large Z_{2} \le z(p_{2})$$ and $$\Large Y_{2} = 0$$ otherwise.

This ensures: $\Large E[Y_{1}] = Pr(Y_{1} = 1) = Pr(Z_{1} \le z(p_{1})) = p_{1},$ $\Large E[Y_{2}] = Pr(Y_{2} = 1) = Pr(Z_{2} \le z(p_{2})) = p_{2},$ $\Large Cov(Y_{1}, Y_{2}) = Pr(Y_{1} = 1, Y_{2} = 1) - p_{1}p_{2}$ $\Large = Pr(Z_{1} \le z(p_{1}), Z_{2} \le z(p_{2})) - p_{1}p_{2}$ $\Large = \Phi[z(p_{1}), z(p_{2}), \rho_{x1x2}] - p_{1}p_{2}$ $\Large = \rho_{y1y2}\sqrt{p_{1}(1 - p_{1})p_{2}(1 - p_{2})},$ $\Large Cor(Y_{1}, Y_{2}) = Cov(Y_{1}, Y_{2})/\sqrt{p_{1}(1 - p_{1})p_{2}(1 - p_{2})} = \rho_{y1y2}.$

Otherwise, ordnorm is called for each pair. If the resulting intermediate matrix is not positive-definite, this is corrected for later.

### Continuous Variables:

Correlations are computed pairwise. findintercorr_cont is called for each pair.

### Continuous-Ordinal Pairs:

findintercorr_cont_cat is called to calculate the intermediate MVN correlation for all Continuous and Ordinal combinations.

Now the two methods will be contrasted.

## Overview of Correlation Method 1:

The intermediate correlations used in correlation method 1 are more simulation based than those in correlation method 2, which means that accuracy increases with sample size and the number of repetitions (see findintercorr). Specifying the seed allows for reproducibility. In addition, method 1 differs from method 2 in the following ways:

1. The intermediate correlation for count variables is based on the method of Yahav & Shmueli (2012), which uses a simulation based, logarithmic transformation of the target correlation. This method becomes less accurate as the variable mean gets closer to zero.

1. Poisson variables: findintercorr_pois is called to calculate the intermediate MVN correlation for all variables.
2. Negative Binomial variables: findintercorr_nb is called to calculate the intermediate MVN correlation for all variables.
2. The ordinal - count variable correlations are based on an extension of the method of Amatya & Demirtas (2015), in which the correlation correction factor is the product of the upper Frechet-Hoeffding bound on the correlation between the count variable and the normal variable used to generate it and a simulated upper bound on the correlation between an ordinal variable and the normal variable used to generate it (see Demirtas and Hedeker (2011)).

1. Poisson variables: findintercorr_cat_pois is called to calculate the intermediate MVN correlation for all variables.
2. Negative Binomial variables: findintercorr_cat_nb is called to calculate the intermediate MVN correlation for all variables.
3. The continuous - count variable correlations are based on an extension of the methods of Amatya & Demirtas (2015) and Demirtas et al. (2012), in which the correlation correction factor is the product of the upper Frechet-Hoeffding bound on the correlation between the count variable and the normal variable used to generate it and the power method correlation between the continuous variable and the normal variable used to generate it (see Headrick and Kowalchuk (2007)). The intermediate correlations are the ratio of the target correlations to the correction factor.

1. Poisson variables: findintercorr_cont_pois is called to calculate the intermediate MVN correlation for all variables.
2. Negative Binomial variables: findintercorr_cont_nb is called to calculate the intermediate MVN correlation for all variables.

### Simulation Process:

The algorithm used in the simulation function rcorrvar that employs correlation method 1 is as follows:

1. Preliminary checks on the distribution parameters and target correlation matrix rho are performed to ensure they are of the correct dimension, format, and/or sign. This function does NOT verify the feasibility of rho, given the distribution parameters. That should be done first using valid_corr, which checks if rho is within the feasible bounds and returns the lower and upper correlation limits.

2. The constants are calculated for the continuous variables using find_constants. If no solutions are found that generate valid power method pdfs, the function will return constants that produce invalid pdfs (or a stop error if no solutions can be found). Errors regarding constant calculation are the most probable cause of function failure. Possible solutions include:

1. changing the seed, or
2. using a list Six of sixth cumulant correction values (if method = “Polynomial”).
3. The support is created for the ordinal variables (if no support provided).

4. The intermediate correlation matrix Sigma is calculated using findintercorr. Note that this will return a matrix that is not positive-definite. If so, there will be a message that it may not be possible to produce variables with the desired distributions. Also, the algorithm of Higham (2002) is used (see Matrix::nearPD) to produce the nearest positive-definite matrix and a message is given.

5. k <- k_cat + k_cont + k_pois + k_nb multivariate normal variables ($$\Large X_{nxk}$$) with correlation matrix Sigma are generated using eigen-value and spectral value decompositions on a $$\Large MVN_{nxk}(0,1)$$ matrix.

6. The variables are generated from $$\Large X_{nxk}$$ using the appropriate transformations (see Variable Types vignette)

7. The final correlation matrix is calculated, and the maximum error (maxerr) from the target correlation matrix is found.

8. If the error loop is specified (error_loop = TRUE), it is used on each variable pair to correct the final correlation until it is within epsilon of the target correlation or the maximum number of iterations has been reached. Additionally, if the extra correction is specified(extra_correct = TRUE), if the maximum error within each variable pair is still greater than 0.1, the intermediate correlation is set equal to the target correlation (with the assumption that the calculated final correlation will be less than 0.1 away from the target).

9. Summary statistics are calculated by variable type.

## Overview of Correlation Method 2:

The intermediate correlations used in correlation method 2 are less simulation based than those in correlation method 1 (see findintercorr2). Their calculations involve greater utilization of correction loops which make iterative adjustments until a maximum error has been reached (if possible). In addition, method 2 differs from method 1 in the following ways:

1. The intermediate correlations involving count variables are based on the methods of Barbiero & Ferrari (2012; 2015). The Poisson or Negative Binomial support is made finite by removing a small user-specified value (i.e. 1e-06) from the total cumulative probability. This truncation factor may differ for each count variable (see max_count_support). The count variables are subsequently treated as ordinal and intermediate correlations are calculated using the correction loop of ordnorm.

2. The continuous - count variable correlations are based on an extension of the method of Demirtas et al. (2012), and the count variables are treated as ordinal. The correction factor is the product of the power method correlation between the continuous variable and the normal variable used to generate it (see Headrick and Kowalchuk (2007)) and the point-polyserial correlation between the ordinalized count variable and the normal variable used to generate it (see Olsson, Drasgow, and Dorans (1982)). The intermediate correlations are the ratio of the target correlations to the correction factor.

1. Poisson variables: findintercorr_cont_pois2 is called to calculate the intermediate MVN correlation for all variables.
2. Negative Binomial variables: findintercorr_cont_nb2 is called to calculate the intermediate MVN correlation for all variables.

### Simulation Process:

The algorithm used in the simulation function rcorrvar2 that employs correlation method 2 is similar to that described for rcorrvar, with a few modifications:

1. The feasibility of rho, given the distribution parameters, should be checked first using the function valid_corr2, which checks if rho is within the feasible bounds and returns the lower and upper correlation limits.

2. After the support is created for the ordinal variables (if no support is provided), the maximum support for the count variables is determined using max_count_support, given truncation value vector pois_eps for Poisson variables and/or nb_eps for Negative Binomial variables. The cumulative probability truncation value may differ by variable, but a good value is $$0.0001$$. The resulting supports and distribution parameters are used to create marginal lists, consisting of the cumulative probabilities for each count variable.

3. The intermediate correlation matrix Sigma is calculated using findintercorr2.

## References

Amatya, A, and H Demirtas. 2015. “Simultaneous Generation of Multivariate Mixed Data with Poisson and Normal Marginals.” Journal of Statistical Computation and Simulation 85 (15): 3129–39. doi:10.1080/00949655.2014.953534.

Barbiero, A, and P A Ferrari. 2015. “Simulation of Correlated Poisson Variables.” Applied Stochastic Models in Business and Industry 31: 669–80. doi:10.1002/asmb.2072.

Demirtas, H, and D Hedeker. 2011. “A Practical Way for Computing Approximate Lower and Upper Correlation Bounds.” The American Statistician 65 (2): 104–9. doi:10.1198/tast.2011.10090.

Demirtas, H, D Hedeker, and R J Mermelstein. 2012. “Simulation of Massive Public Health Data by Power Polynomials.” Statistics in Medicine 31 (27): 3337–46. doi:10.1002/sim.5362.

Emrich, L J, and M R Piedmonte. 1991. “A Method for Generating High-Dimensional Multivariate Binary Variates.” The American Statistician 45: 302–4. doi:10.1080/00031305.1991.10475828.

Ferrari, P A, and A Barbiero. 2012. “Simulating Ordinal Data.” Multivariate Behavioral Research 47 (4): 566–89. doi:10.1080/00273171.2012.692630.

Headrick, T C, and R K Kowalchuk. 2007. “The Power Method Transformation: Its Probability Density Function, Distribution Function, and Its Further Use for Fitting Data.” Journal of Statistical Computation and Simulation 77: 229–49. doi:10.1080/10629360600605065.

Higham, N. 2002. “Computing the Nearest Correlation Matrix - a Problem from Finance.” IMA Journal of Numerical Analysis 22 (3): 329–43. doi:10.1093/imanum/22.3.329.

Olsson, U, F Drasgow, and N J Dorans. 1982. “The Polyserial Correlation Coefficient.” Psychometrika 47 (3): 337–47. doi:10.1007/BF02294164.

Yahav, I, and G Shmueli. 2012. “On Generating Multivariate Poisson Data in Management Science Applications.” Applied Stochastic Models in Business and Industry 28 (1): 91–102. doi:10.1002/asmb.901.