What is what metric-dimension combination is not valid?

It is difficult to provide a specific answer without knowing the context of the question, as the validity of a metric-dimension combination depends on the specific use case. However, in general, any combination of metrics and dimensions that do not align with the business goals or are not meaningful in the analysis of the data can be considered invalid. For example, measuring the number of page views by weather condition (a dimension) may not provide useful insights for a retail company looking to increase sales, as there may not be a clear correlation between the two metrics. Similarly, tracking the number of social media followers (a metric) by age group (a dimension) may not be useful for a healthcare organization focused on patient outcomes. It is important to choose metrics and dimensions that are relevant to the business goals and provide actionable insights.