add_min_set_objective: Add minimum set objective

In prioritizr/ppr: Optimal Project Prioritization

Add minimum set objective

Description

Set the objective of a project prioritization problem() to minimize the cost of the solution whilst ensuring that all targets are met. This objective is conceptually similar to that used in Marxan (Ball, Possingham & Watts 2009).

Usage

add_min_set_objective(x)

Arguments

x	ProjectProblem object.

Details

A problem objective is used to specify the overall goal of the project prioritization problem. Here, the minimum set objective seeks to find the set of actions that minimizes the overall cost of the prioritization, while ensuring that the funded projects meet a set of persistence targets for the conservation features (e.g. populations, species, ecosystems). Let I represent the set of conservation actions (indexed by i). Let C_i denote the cost for funding action i. Also, let F represent each feature (indexed by f), T_f represent the persistence target for feature f, and E_f denote the probability that each feature will go extinct given the funded conservation projects.

To guide the prioritization, the conservation actions are organized into conservation projects. Let J denote the set of conservation projects (indexed by j), and let A_{ij} denote which actions i in I comprise each conservation project j in J using zeros and ones. Next, let P_j represent the probability of project j being successful if it is funded. Also, let B_{fj} denote the enhanced probability that each feature f in F associated with the project j in J will persist if all of the actions that comprise project j are funded and that project is allocated to feature f. For convenience, let Q_{fj} denote the actual probability that each f in F associated with the project j in J is expected to persist if the project is funded. If the argument to adjust_for_baseline in the problem function was set to TRUE, and this is the default behavior, then Q_{fj} = (P_j B_{fj}) + ((1 - (P_j B_{fj})) * (P_n \times B_{fn})), where n corresponds to the baseline "do nothing" project. This means that the probability of a feature persisting if a project is allocated to a feature depends on (i) the probability of the project succeeding, (ii) the probability of the feature persisting if the project does not fail, and (iii) the probability of the feature persisting even if the project fails. Otherwise, if the argument is set to FALSE, then Q_{fj} = P_{j} * B_{fj}.

The binary control variables X_i in this problem indicate whether each project i in I is funded or not. The decision variables in this problem are the Y_{j}, Z_{fj}, and E_f variables. Specifically, the binary Y_{j} variables indicate if project j is funded or not based on which actions are funded; the binary Z_{fj} variables indicate if project j is used to manage feature f or not; and the semi-continuous E_f variables denote the probability that feature f will go extinct.

Now that we have defined all the data and variables, we can formulate the problem. For convenience, let the symbol used to denote each set also represent its cardinality (e.g. if there are ten features, let F represent the set of ten features and also the number ten).

Maximize sum_i^I C_i X_i (eqn 1a); Subject to: E_f <= T_f for all f in F (eqn 1b), E_f = 1 - sum_j^J Y_{fj} Q_{fj} for all f in F (eqn 1c), Z_{fj} <= Y_j for all j in J (eqn 1d), sum_j^J Z_{fj} * ceil(Q_{fj}) = 1 for all f in F (eqn 1e), A_{ij} Y_{j} <= X_{i} for all i I, j in J (eqn 1f), E_f >= 0, E_f <= 1 for all f in F (eqn 1g), X_i, Y_j, Z_{fj} in [0, 1] for all i in I, j in J, f in F (eqn 1h)

The objective (eqn 1a) is to minimize the cost of the funded actions. Constraints (eqn 1b) ensure that the persistence targets are met. Constraints (eqn 1c) calculate the probability that each feature will go extinct according to their allocated project. Constraints (eqn 1d) ensure that feature can only be allocated to projects that have all of their actions funded. Constraints (eqn 1e) state that each feature can only be allocated to a single project. Constraints (eqn 1f) ensure that a project cannot be funded unless all of its actions are funded. Constraints (eqns 1g) ensure that the probability variables (E_f) are bounded between zero and one. Constraints (eqns 1h) ensure that the action funding (X_i), project funding (Y_j), and project allocation (Z_{fj}) variables are binary.

Value

ProjectProblem object with the objective added to it.

References

Ball IR, Possingham HP & Watts M (2009) Marxan and relatives: software for spatial conservation prioritisation. Spatial conservation prioritisation: Quantitative methods and computational tools, 185-195.

See Also

objectives, targets.

Examples

# load the ggplot2 R package to customize plot
library(ggplot2)

# load data
data(sim_projects, sim_features, sim_actions)

# build problem with minimum set objective and targets that require each
# feature to have a 30% chance of persisting into the future
p <- problem(sim_projects, sim_actions, sim_features,
             "name", "success", "name", "cost", "name") %>%
      add_min_set_objective() %>%
      add_absolute_targets(0.3) %>%
      add_binary_decisions()

## Not run: 
# solve problem
s <- solve(p)

# print solution
print(s)

# plot solution, and add a dashed line to indicate the feature targets
plot(p, s) +
geom_hline(yintercept = 0.3, linetype = "dashed")

## End(Not run)

prioritizr/ppr documentation built on Sept. 10, 2022, 1:18 p.m.