---
title: "Signed Two-Mode Networks"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{05 Signed Two-Mode Networks}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r, include = FALSE}
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>",
  message=FALSE,
  warning=FALSE
)
```

This vignette describes methods implemented to analyze signed two-mode networks.

```{r setup}
library(igraph)
library(signnet)
```

## Projections

A common analytic tool for two-mode networks is to project the network onto on relevant mode.
This is easily done using the adjacency matrix $A$. $AA^T$ yields the row projection and $A^TA$ the column
projection. The resulting networks will thus be weighted. Several methods exist to turn a weighted projection
into an unweighted network where only the most significant edges are included. A number of these methods are implemented in the [backbone](https://cran.r-project.org/package=backbone) package.

Projecting signed networks, however, is not as straightforward. Consider the following simple example.

```{r simple s2mode}
el <- matrix(c(1,"a",1,"b",1,"c",2,"a",2,"b"),ncol = 2,byrow = TRUE)
g <- graph_from_edgelist(el,directed = FALSE)
E(g)$sign <- c(1,1,-1,1,-1)
V(g)$type <- c(FALSE,TRUE,TRUE,TRUE,FALSE)
```

```{r graph, echo=FALSE,out.width = "50%",fig.align='center'}
knitr::include_graphics("small_signed2mode.png")
```

If we use the regular projection rules we obtain
```{r matmul}
A <- as_incidence_signed(g)
R <- A%*%t(A)
C <- t(A)%*%A
R
C
```

The row projection suggests that there is no relation between 1 and 2, when in fact there is a negative path (via b) and
a positive path (via a) between them. The same holds for the column projection and the nodes a and b.

The paper of Schoch introduces two projection methods that circumvent this "nullification". The package implements the *duplication* approach since it plays well with existing binarization tools. The first stepp is to turn the signed two-mode network into an unsigned one.
This is done by duplicating all vertices of the primary mode (i.e. the one to project on). For example, vertex a turns into two
vertices "a-pos" and "a-neg". The vertices of the secondary mode connect to these new vertices depending on the sign of edge.
For instance, 1 has a positive edge to a and thus 1 connects to a-pos.

This can be done for the whole network with the function `as_unsigned_2mode()` by specifying the primary mode (either TRUE or FALSE).
```{r duplicate}
gu <- as_unsigned_2mode(g,primary = TRUE)
gu
```

Now, any binarization toll (e.g. from the `backbone` package) can be applied since the network is an unsigned
two-mode network. For illustration, we include all edges with a weight greater one (the "universal" approach) since it can be
done without the `backbone` package.

```{r binarize}
pu <- bipartite_projection(gu,which = "true")
pu <- delete_edge_attr(pu,"weight")
pu
```

After binarization, the network is turned back to an unsigned network using a *contraction rule*. 
The contraction rule works as follows. 

If there is an edge (a-pos,b-pos) or (a-neg,b-neg) in the projection
then there is a positive edge (a,b) in the signed projection. 

If there is an edge (a-pos,b-neg) or (a-neg,b-pos) in the projection
then there is a negative edge (a,b) in the signed projection.

If there is an edge (a-pos,b-pos) **and** (a-neg,b-pos) (or, e.g., (a-neg,b-neg) **and** (a-pos,b-neg)) in the projection
then there is an *ambivalent edge* (a,b) in the signed projection.

This is done with the function `as_signed_proj()`.
```{r contract}
ps <- as_signed_proj(pu)
as_data_frame(ps,"edges")
```

The projection of a signed two-mode network thus may contain three types of edges (positive ("P"), negative ("N") or ambivalent ("A")).
The concept of ambivalent ties comes from work by Abelson & Rosenberg and Cartwright & Harary.

More technical details can be found in the original paper by Schoch.
Consult the vignette about [complex matrices](complex_matrices.html) to learn about analyzing signed networks with ambivalent ties.

## References

Doreian, Patrick, Paulette Lloyd, and Andrej Mrvar. 2013. "Partitioning Large Signed Two-Mode Networks: Problems and Prospects." Social Networks, Special Issue on Advances in Two-mode Social Networks, 35 (2): 178–203. 

Schoch, David. 2020. "Projecting Signed Two-Mode Networks" Mathematical Sociology, forthcoming

Abelson, Robert P., and Milton J. Rosenberg. 1958. “Symbolic Psycho-Logic: A Model of Attitudinal Cognition.” Behavioral Science 3 (1): 1–13. 

Cartwright, Dorwin, and Frank Harary. 1970. “Ambivalence and Indifference in Generalizations of Structural Balance.” Behavioral Science 15 (6). 

