---
title: "Aggregating Matching Variables Over Time"
author: "Michael Robbins and Steven Davenport"
date: "`r Sys.Date()`"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{Aggregating Matching Variables Over Time}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

This is a brief discussion of how microsynth allows the user to aggregate
outcome variables (passed to `match.out` or `match.out.min`) over time, either
before or after matching, and how that can affect the workings of two other
arguments: `period` and `start.pre`.

## Declaring outcome variables as a vector or list

microsynth will match treatment and control based on the variables passed to
`match.out` and `match.out.min`. But there are two different ways to input
parameters to `match.out`, and they have different implications for how the
arguments work.

The simple way to pass variables to match.out is with a simple vector of
variable names (e.g., `match.out=c("Y1", "Y2")`. A more complex way is to pass a
list, containing a set of vectors with named corresponding to variable names and
elements corresponding to instructions for how to aggregate those variables over
time before matching, e.g. `match.out = list('Y1' = c(1, 3, 3), 'Y2'=
c(2,5,1))`. Under this specification, if the intervention occurs at time 10,
then matching will proceed across a) The value of Y1 at time 10; b) the sum of
Y1 across times 7, 8 and 9; c) the sum of Y1 across times 4, 5 and 6; e) The sum
of Y2 across times time 9 and 10; e) the sum of Y2 across times 4, 5, 6, 7, and
8; f) the value of Y2 at time 3.

## Interactions with other arguments

The choice between the simple and the complex input method is important, and it
alters how other arguments work.

`period` specifies the granularity of the data used by microsynth. When provided
simple input from match.out (i.e., a vector of variable names),  `period`
indicates that those variables should be aggregated into the duration equal to
the value of `period`.  E.g., if data are provided monthly and `period=3`, then
variables will be aggregated to the year-quarter before matching. But when
provided complex input from match.out (i.e., a list of names and integers), then
the user has already manually specified how the variables should be aggregated
prior to matching; in this case, `period` will only affect the appearance of
charts and how results are reported (but not the matching procedure).

`start.pre` is also affected. When match.out is provided the simple input (a
vector of variable names), start.pre will identify the time of the beginning of
the pre-intervention data to be used for matching, plotting, and displaying
results. Data observed any time before the `start.pre` will be ignored
entirely. If provided the complex (list) input, then `start.pre` will merely
affect plotting behavior and suppress some of the results. But observations
before the `start.pre` will still be used for matching.

In conclusion, passing a list to `match.out` will lock into place the parameters
around the matching procedure; arguments such as `period` and `start.pre` will
only alter how results are reported and how charts are plotted. But when
`match.out` is passed only variable names, then `period` and `start.pre` will
also govern the matching procedure.

