The attribution model that describes how to distribute credit for a particular conversion across potentially many prior interactions.

[Proto definition ![GitHub](https://developers.google.com/site-assets/logo-github.svg)](https://github.com/googleapis/googleapis/blob/master/google/ads/googleads/v26/enums/attribution_model.proto)

| Enums ||
|---|---|
| ## `EXTERNAL` | Uses external attribution. |
| ## `GOOGLE_ADS_LAST_CLICK` | Attributes all credit for a conversion to its last click. |
| ## `GOOGLE_SEARCH_ATTRIBUTION_DATA_DRIVEN` | Flexible model that uses machine learning to determine the appropriate distribution of credit among clicks using Google Search attribution. |
| ## `GOOGLE_SEARCH_ATTRIBUTION_FIRST_CLICK` | Attributes all credit for a conversion to its first click using Google Search attribution. |
| ## `GOOGLE_SEARCH_ATTRIBUTION_LINEAR` | Attributes credit for a conversion equally across all of its clicks using Google Search attribution. |
| ## `GOOGLE_SEARCH_ATTRIBUTION_POSITION_BASED` | Attributes 40% of the credit for a conversion to its first and last clicks. Remaining 20% is evenly distributed across all other clicks. This uses Google Search attribution. |
| ## `GOOGLE_SEARCH_ATTRIBUTION_TIME_DECAY` | Attributes exponentially more credit for a conversion to its more recent clicks using Google Search attribution (half-life is 1 week). |
| ## `UNKNOWN` | Used for return value only. Represents value unknown in this version. |
| ## `UNSPECIFIED` | Not specified. |