# Linear mixed models in lmt

### Matrix notation, factors and sub-factors

Consider the multi-variate linear mixed model

$$ \left( \begin{array}{c} y_1 \\ y_2 \\ y_3 \end{array} \right) = \left( \begin{array}{ccc} X_1 & 0 & 0 \\ 0 & X_2 & 0 \\ 0 & 0 & X_3 \end{array} \right) \left( \begin{array}{c} b_1 \\ b_2 \\ b_3 \end{array} \right) + \left( \begin{array}{ccc} Z_1 & 0 & 0\\ 0 & Z_2 & 0\\ 0 & 0 & Z_3 \end{array} \right) \left( \begin{array}{c} u_1 \\ u_2 \\ u_3 \end{array} \right) + \left( \begin{array}{c} e_1 \\ e_2 \\ e_3 \end{array} \right) $$

where $$(y_1,y_2,y_3)'$$, $$(b_1,b_2,b_3)'$$, $$(u_1,u_2,u_3)'$$ and $$(e_1,e_2,e_3)'$$ are vectors of response variables, effects of fixed factors, effects of random factors and effects of residuals respectively, and matrices $$\left( \begin{array}{ccc} X_1 & 0 & 0 \\ 0 & X_2 & 0 \\ 0 & 0 & X_3 \end{array} \right)$$, and $$ \left( \begin{array}{ccc} Z_1 & 0 & 0\\ 0 & Z_2 & 0\\ 0 & 0 & Z_3 \end{array} \right) $$ are block-diagonal design matrices linking effects in the respective vectors to their related response variables. In usual mixed model terminology $$b_1$$, $$b_2$$ and $$b_3$$ are called fixed factors, and $$u_1$$, $$u_2$$ and $$u_3$$ are called random factors. Ignoring the residual the above model has in total 6 factors.

However, the model maybe rewritten in matrix formulation as

$$vec(Y)=Xvec(B)+Zvec(U)+vec(E)$$,

where $$vec$$ is the vectorization operator, $$Y=[y_1,y_2,y_3]$$, $$B=[b_1,b_2,b_3]$$, $$U=[u_1,u_2,u_3]$$ and $$E=[e_1,e_2,e_3]$$ are column matrices of response variables, the effects of the fixed and random factor, and the residuals, respectively, and $$X=\left( \begin{array}{ccc} X_1 & 0 & 0 \\ 0 & X_2 & 0 \\ 0 & 0 & X_3 \end{array} \right)$$, and $$Z= \left( \begin{array}{ccc} Z_1 & 0 & 0\\ 0 & Z_2 & 0\\ 0 & 0 & Z_3 \end{array} \right) $$. The distribution assumption for the random components in the model are $$vec(U^{'})\sim N((0,0,0)',\Gamma_u \otimes \Sigma_u)$$ and $$vec(E^{'})\sim N((0,0,0)',\Gamma_e \otimes \Sigma_e)$$. Note that the column and row dimensions of $$U$$ are determined by the column dimension of $$\Sigma_u$$ and $$\Gamma_u$$ respectively.

Slightly different to the above terminology, **lmt** refers to $$B$$ and $$U$$ as factors, and therefore the model has only two factors, whereas the columns in $$B$$ and $$U$$ are referred to as **sub-factors**.

Following the above matrix notation **lmt** will always invoke only one factor for all modelled fixed classification variables and only one factor for all modelled fixed continuous co-variables. Sub-factors are summarized into a single random factors if they share the same $$\Sigma$$ matrix. Thus, **lmt** will invoke as many random factors as there are different $$\Gamma \otimes \Sigma$$ constructs. That is, in **lmt** terminology the multi-variate model

$$ \left( \begin{array}{c} y_1 \\ y_2 \\ y_3 \end{array} \right) = \left( \begin{array}{ccc} X_1 & 0 & 0 \\ 0 & X_2 & 0 \\ 0 & 0 & X_3 \end{array} \right) \left( \begin{array}{c} b_1 \\ b_2 \\ b_3 \end{array} \right) + \left( \begin{array}{cccccc} Z_{d,1} & 0 & 0 & Z_{m,1} & 0 & 0\\ 0 & Z_{d,2} & 0 & 0 & Z_{m,2} & 0\\ 0 & 0 & Z_{d,3} & 0 & 0 & Z_{m,3}\\ \end{array} \right) \left( \begin{array}{c} u_{d,1} \\ u_{d,2} \\ u_{d,3} \\ u_{m,1} \\ u_{m,2} \\ u_{m,3} \end{array} \right) + \left( \begin{array}{ccc} W_1 & 0 & 0\\ 0 & W_2 & 0\\ 0 & 0 & W_3 \end{array} \right) \left( \begin{array}{c} v_1 \\ v_2 \\ v_3 \end{array} \right) + \left( \begin{array}{c} e_1 \\ e_2 \\ e_3 \end{array} \right) $$

with $$(u_{d,1},u_{d,2},u_{d,3},u_{m,1},u_{m,2},u_{m,3})'\sim N((0,0,0,0,0,0)',\Sigma_u \otimes \Gamma_u)$$ and $$(v_1,v_2,v_3)'\sim N((0,0,0)',\Sigma_v \otimes \Gamma_v)$$, rewritten as $$vec(Y)=Xvec(B)+Zvec(U)+Wvec(V)+vec(E)$$ will have only 3 factors, $$B$$, $$U$$ and $$V$$ with $$b_1,b_2,b_3$$, $$u_{d,1},u_{d,2},u_{d,3},u_{m,1},u_{m,2},u_{m,3}$$ and $$v_1,v_2,v_3$$ being subfactors of $$U$$ and $$V$$ respectively.

### Model syntax

Thee syntax for communicating the model to **lmt** is effectively: "**just write the model**", where the model is written into the space of a particular xml element of the parameter file.

Note that **lmt** will only check whether the specified model can be built, not whether the model is meaningful or allows for a statistical inference.

#### An example

An example for a valid **lmt** model string would be

y=x*b+age(t(co(p(1,2))))*c+id*u(v(my_var(1))).

The components of the model string are

- the response variable y , which must be a column name in the data file
- model variables x , age and id , which must be a column names in the data file
- sub-factors b , c and u which are user-defined alpha-numeric character strings
- relation operators = , * and +
- a variable specifier (t(co(p(1,2)))) used to provide further information about variable age
- a sub-factor specifier (v(my_var(1))) used to provide further information about sub-factor u

#### Variables

Response variables and model variables are named using user-defined alpha-numeric character strings. These character strings must be column names in the data file. There is no size limitation for the variables names.

Model variables can be used in several equations. That is, the bi-variate model

y1=x*b1 y2=x*b2,

with variables x, y1 and y2 being column names in the data file, is equivalent to the model $$ \left( \begin{array}{c} y_1 \\ y_2 \end{array} \right)= \left( \begin{array}{cc} X & 0 \\ 0 & X \end{array} \right) \left( \begin{array}{c} b_1 \\ b_2 \end{array} \right)+ \left( \begin{array}{c} e_1 \\ e_2 \end{array} \right) $$

#### Sub-factors

Sub-factors are named using user-defined alpha-numeric character strings. There is no size limitation for the variables names.

Contrarily to using variables across traits, using a sub-factor across traits changes the model. That is model

y1=x*b1 y2=x*b1

is equivalent to $$ \left( \begin{array}{c} y_1 \\ y_2 \end{array} \right)= \left( \begin{array}{c} X \\ X \end{array} \right) \begin{array}{c} b_1 \\ \end{array} + \left( \begin{array}{c} e_1 \\ e_2 \end{array} \right) $$

#### Relational operators

The rules for using relational operators are

- = links a single response variable to the model
- * links a model variable to it's sub-factor, which together form a model term
- + concatenates different model terms.

#### Specifiers

Variables and sub-factors maybe accompanied by a specifier. A specifier is a tree diagramm in Newick format with all nodes named, but leaf nodes can be unnamed, and where the root node name is the variable or sub-factor name. The specifier provides additional information about a variable or sub-factor. The **lmt** version of the above tree diagram differs in that

- the parent nodes precede child nodes
- child nodes within the same parent node are separated by semicolon
- sibling nodes can be mutually exclusive, that is only one sibling node maybe allowed
- leaf nodes maybe not named but contain additional, maybe comma-separated information
- if a child node is marked as default, the child node and it's immediate parent node can be omitted
- if a node is marked as optional it can be omitted. if an optional node is used it's compulsory child nodes must be included

##### Variable specifiers

Variable specifiers are used to communicate further information which may be that the variable

- is continuous but real numbers
- is continuous but integer numbers
- is a genetic group regression matrix
- undergoes a polynomial expansion
- is associated to a nesting variable

etc.

The tree diagram for the variable specifier is

with the nested parentheses representation written as

variable(t(co(t(i;r);p(polynomial ids);n(variable));cl;gg(pedigree)))

Note that the above representation would **not** yield a valid specifier useable in an equation as the diagram contains sibling nodes which are mutually exclusive, thus allowing only one of the sibling node to occur in the specifier. That is valid specifiers would be

variable(t(cl))variable(t(co))variable(t(gg(pedigree)))variable(t(co(t(r);p(polynomial id);n(variable))))variable(t(co(t(i);p(polynomial id);n(variable))))

Since a default-determining leave node and it's immediate parent node can be omitted the following equality holds:

variable(t(cl))=variablevariable(t(co(t(r))))=variable(t(co))

A variable can have several different specifiers. Notwithstanding the statistical soundness, an example would be

y=x(t(co))*b + x(t(co(n(g))))*c

where y, x and g are columns names in the data file. The equivalent model would be $$ y=D_x ib+D_x X_gc + e $$ where $$D_x$$ is a diagonal matrix constructed from x , $$i$$ is a vector of ones, $$X_g$$ is a design matrix constructed from classification variable $$g$$, and $$b$$ and $$c$$ are the sub-factors.

##### Sub-factor specifiers

Sub-factor specifiers are used to communicate further information which may be that the sub-factor

- is a random sub-factor
- to which variance it is related to
- which diagonal element in the $$\Sigma$$ matrix of the variance it is related to

The tree diagram for the sub-factor specifier is

with the nested parentheses representation written as

sub-factor(v(variance name(diagonal position)))

According to the tree diagram the sub-factor specifier in the example model string translates to

- sub-factor name is u
- the sub-factor has a variance
- variance name is my_var
- the diagonal position in $$\Sigma$$ is #1.

Contrarily to variables, a sub-factor can have only one specifier assigned. That is, **lmt** would not accept a bi-variate model

y1=mu*b1+id*u1(v(myvar(1)) y2=mu*b2+id*u1(v(myvar(2))

because the two specifiers assigned to u1 , (v(myvar(1)) and (v(myvar(2) differ.

#### Polynomials

**lmt** allows for random and fixed regression on polynomials of the general form $$y=f(x)b+e$$, where $$y$$ is the response variable, $$x$$ is the co-variable, $$f(x)$$ is the polynomial function of $$x$$, and $$e$$ is the residual. Polynomial functions are written into the space of a particular xml element of the parameter file

##### User defined polynomials

Polynomials expressions can be freely defined by the user but must not have more than one variable, where that single variable **must be x**. That is polynomial expression

3*x^2+exp(x)

is a valid, whereas expressions

3*y^2+exp(y) 3*x^2+exp(y)

are not valid as a variable named y is not supported.

##### Hard-coded polynomials

For convenience reasons complex polynomials are hard-coded and can be referenced by their hard-coded abbreviation.

polynomial | abbreviation |
---|---|

Legendre order 0 | l0 |

Legendre order 1 | l1 |

Legendre order 2 | l2 |

Legendre order 3 | l3 |

Legendre order 4 | l4 |

Legendre order 5 | l5 |

Legendre order 6 | l6 |

Legendre order 7 | l7 |

That is, instead of writing `0.5*(5*x^3-3*x)`

one can simply write `l3`

.

##### Referencing polynomials in equations

Including polynomials directly into the equation would have meant that for multi-trait models complex polynomials would have been written repeatedly into the respective variable specifier which is an error prone process. Therefore, as explained in #Variable specifier polynomials are referenced in the variable specifier via using the number of that line inside the parameter file element in which they occur.

##### Polynomial variable expansion and $$\Sigma$$ dimensions in random regression models

If a variable has a variable specifier assigned which contains polynomials, e.g

z(t(co(t(i);p(1,2);n(id)))

and the associated sub-factor has a sub-factor specifier assigned which declares the sub-factor to be random, e.g.

z(t(co(t(i);p(1,2);n(id)))*u(v(myvar(2,3)),

it must be assured that the number of referenced $$\Sigma$$ diagonal elements in the sub-factor specifier is equal to the number of referenced polynomials in the variable specifier. Further, the first referenced polynomial is related to the first referenced diagonal element, the second to the second and so forth.

A miss-fit between the number of referenced polynomials and the number of referenced $$\Sigma$$ diagonal elements will cause an error stop.

**lmt**'s factor naming convention

**lmt** models as many factors as there are independent co-variance structures in the model, a factor for all fixed classification variables, and a factor for all fixed continuous co-variables. These factors are internally named, where the internal names are used in the output files, in output file names, and to override defaults in the parameter file. The convention used to name the factors is:

- "fxco" is the default name for the factor modelling the fixed classification variables and fixed continuous variables
- the names of random factors are user defined where the actual name is that of the variance

### Treatment of fixed continuous variables

**lmt** will center fixed continuous variables. That is, the polynomial $$x+x^2$$ becomes $$x-\overline{x}+(x-\overline{x})^2$$. Nested variables will be centered within their nesting. Centering implies that one should not fit an intercept via $$x^0$$ as it will results in an error message. However, centering can be explicitly switched off in the specifier of each variable.

### Column rank of $$X$$

lmt will try to find and remove linear dependencies in $$X$$. However, the respective computations are applied within trait and include only fixed classification variables. That means fitting classification variables implicitly via applying polynomials of power zero to a continuous co-varibales, or switching off the centering of continuous co-variables, or fitting genetic groups as fixed can lead to linear dependencies in $$X$$ which are not accounted for. Then subsequent operations may fail or results maybe incorrect.