> ## Documentation Index
> Fetch the complete documentation index at: https://docs.graphext.com/llms.txt
> Use this file to discover all available pages before exploring further.

# link_session_items

> Link items (e.g. products) in sessions (baskets) if one item makes the presence of the other in  the same session more likely. 

A link (or association) A->B is created between items A and B if the presence of A makes the presence
of B in the same session N times more likely.

For further details about the algorithm see e.g. [association rule learning](https://en.wikipedia.org/wiki/Association_rule_learning).

## Usage

The following example shows how the step can be used in a recipe.

<Accordion title="Examples" icon="code" defaultOpen="true">
  <Tabs>
    <Tab title="Example 1">
      The following call creates links between pairs of items A and B, if:

      * A occurs in at least 7 sessions
      * B occurs in at least 25% of sessions containing A
      * The presence of A in a session makes the presence of B in the same session at least twice as likely.

      Note that the last condition is equivalent to saying that the overall frequency of B in all sessions must be less than 12.5% (half of 25%). In other words, a minimum lift of 2 means that the frequency of B, in sessions already containing A, must be twice the background frequency of B in general.

      As an example, the percentage of shopping baskets containing milk (item B) may be 10%. However, amongst those baskets already containing cereals, the percentage containing milk is likely to be higher. If milk occured e.g. in 30% of baskets also having cereals, than the lift of the rule cereal->milk would be 3. The buying of cereal make the buying of milk 3 times more likely.

      ```stan theme={null}
      link_session_items(items.id, sessions.item_ids, {
        "min_support": 7,
        "min_confidence": 25,
        "min_lift": 2
      }) -> (items.targets, items.weights)
      ```
    </Tab>

    <Tab title="Signature">
      General syntax for using the step in a recipe. Shows the inputs and outputs the step is expected to receive and will produce respectively. For futher details see sections below.

      ```stan theme={null}
      link_session_items(items: category|number, sessions: list[category]|list[number], {
          "param": value,
          ...
      }) -> (targets: column, weights: column)
      ```
    </Tab>
  </Tabs>
</Accordion>

## Inputs & Outputs

The following are the inputs expected by the step and the outputs it produces. These are generally
columns (`ds.first_name`), datasets (`ds` or `ds[["first_name", "last_name"]]`) or models (referenced
by name e.g. `"churn-clf"`).

<Accordion title="Inputs" icon="right-to-bracket">
  <ParamField path="items" type="column[category|number]" required>
    A column containing the IDs of items to analyze.
  </ParamField>

  <ParamField path="sessions" type="column[list[category]|list[number]]" required>
    A column containing lists of IDs corresponding to items in the same sessions, basket etc.
  </ParamField>
</Accordion>

<Accordion title="Outputs" icon="right-from-bracket">
  <ParamField path="targets" type="column" required>
    A column containing for each item a list of IDs (row numbers) identfying other items it will be linked to.
  </ParamField>

  <ParamField path="weights" type="column" required>
    A column containing for each item a list of weights identfying the "importance" of each link to
    other items identified in the `targets` column.
  </ParamField>
</Accordion>

## Configuration

The following parameters can be used to configure the behaviour of the step by including them in
a json object as the last "input" to the step, i.e. `step(..., {"param": "value", ...}) -> (output)`.

<Accordion title="Parameters" defaultOpen="true" icon="sliders">
  <ParamField path="min_support" type="[number, integer]" default="10">
    Minimum Support.
    Minimum support of a rule antecedent. If it is \< 1 it will be taken as a proportion.
    In any other case it will be expected as a positive integer representing the count.
    Create link A->B only if A occurred in at least this many sessions.

    <Accordion title="Options">
      <Tabs>
        <Tab title="number">
          <ParamField path="{_}" type="number">
            number.

            Values must be in the following range:

            ```javascript theme={null}
            0 < {_} < 1
            ```
          </ParamField>
        </Tab>

        <Tab title="integer">
          <ParamField path="{_}" type="integer">
            integer.

            Values must be in the following range:

            ```javascript theme={null}
            1 ≤ {_} < inf
            ```
          </ParamField>
        </Tab>
      </Tabs>
    </Accordion>
  </ParamField>

  <ParamField path="min_confidence" type="number" default="20">
    Minimum Confidence.
    Expressed as a rule as a percentage.
    Include link A->B only if B occurred in at least this percentage of sessions also containing A.

    Values must be in the following range:

    ```javascript theme={null}
    0 ≤ min_confidence ≤ 100
    ```
  </ParamField>

  <ParamField path="min_lift" type="[number, null]">
    Minimum Lift.
    Expressed as multipler/ratio. Include link A->B only if A makes the presence of B in the same
    sessions at least this many times more likely.
  </ParamField>

  <ParamField path="weight_metric" type="string" default="rule_lift_pct">
    Metric for link weight.

    Values must be one of the following:

    `itemset_support_abs` `itemset_support_pct` `filter_metric_abs` `filter_metric_pct` `antecedent_support_abs` `antecedent_support_pct` `consequent_support_abs` `consequent_support_pct` `rule_confidence_pct` `rule_lift_abs` `rule_lift_pct`
  </ParamField>
</Accordion>