Cost Per Mille (CPM)

CPI stands for Cost Per Install, meaning the cost that an advertiser pays each time a user installs their mobile app after clicking on an ad. CPI is a performance-based pricing model, which means that the advertiser only pays for results that directly contribute to their business goals, such as app installations. This pricing model is particularly popular among mobile app developers who want to promote their apps and acquire new users.

Cost Per Action (CPA) is a performance-based pricing model in which an advertiser pays only when a user completes a specific post-install action, such as making a purchase, starting a free trial, or subscribing to a service. Unlike CPI, which measures cost at the install level, CPA ties advertising spend directly to business outcomes further down the funnel. For subscription apps, common CPA events include trial starts, first subscription payments, or registration completions. CPA is particularly useful for evaluating the true efficiency of user acquisition campaigns, since an install alone doesn't generate revenue — what matters is whether the user converts into a paying subscriber. By optimizing toward CPA rather than CPI, growth teams can focus ad spend on channels and creatives that deliver users with higher intent and better downstream conversion behavior, even if those channels have a higher cost per install on the surface.

Impression-to-install rate (also called IPM — Installs Per Mille, or installs per thousand impressions) measures the percentage of ad impressions that ultimately result in an app install. It is calculated by dividing the number of installs by the number of ad impressions and multiplying by 100 (or by 1,000 for the IPM variant). This metric combines ad engagement efficiency (do people click?) with post-click conversion efficiency (do people install after clicking?), making it a holistic measure of creative and targeting quality. A high impression-to-install rate indicates that the ad creative resonates with the target audience and that the app store listing or landing page effectively converts interested users. Typical impression-to-install rates vary significantly by ad format and vertical, with video ads and rewarded formats generally outperforming static banners. Monitoring this rate across different creatives, audiences, and placements helps growth teams identify their most efficient combinations and detect creative fatigue early.

Ad servers play a crucial role in housing a campaign's various creative elements like images, audio, and videos, and deciding which versions to present to particular audiences. Additionally, advertisement servers collect essential information on ad performance, such as impressions and clicks, which can offer valuable insights to a UA manager.

Creative fatigue occurs when an advertising audience has been exposed to the same ad creative too many times, resulting in declining click-through rates, rising cost per install, and diminishing campaign performance. In mobile user acquisition, creative fatigue is one of the most common reasons for campaign degradation over time. As users see the same images, videos, or messaging repeatedly, they begin to ignore the ads — a phenomenon sometimes called "ad blindness." Detecting creative fatigue early is critical for maintaining efficient UA spend. Key indicators include a decline in CTR while impressions remain stable, rising CPIs, and lower conversion rates from impression to install. Growth teams combat creative fatigue by maintaining a pipeline of fresh creatives, systematically rotating ad variations, testing new concepts and formats, and segmenting audiences to control frequency exposure. Apps running web funnel campaigns face similar dynamics — landing page designs and value propositions must be periodically refreshed to sustain conversion rates.