What type of study measures relative impact of different sources of advertising over time?

The correct answer is Marketing Mix Models (MMM) because these studies are specifically designed to measure the relative impact of various advertising sources over time, allowing marketers to understand how different channels contribute to overall performance and sales. By analyzing historical data and identifying patterns in consumer behavior, MMM provides insights into the effectiveness of each advertising source, helping advertisers optimize their media spending and strategy. In contrast, brand reach focuses on the extent of audience exposure to a brand, while Mobile Measurement Partners (MMPs) primarily track mobile app performance and attribution, making MMM the most relevant option for assessing the comparative impact of advertising sources.