discussion on the relative merits of peer review and metrics.

The various assessment exercises carried out in the UK during the past decades have all been accompanied by papers that compare citation metrics and peer review. Although the results vary from field to field, most studies found correlations of about $0.7$. Some authors obtained higher correlations, on the order of $0.9$. However, the *Metric Tide* report found significantly lower correlations in the range of about $0.2$–$0.4$. Interestingly, even when authors obtained similar correlations, they did not always draw the same conclusion. Some, such as @Mryglod2015a and @Mahdi2008, argued that a correlation of $0.7$ is too low to consider using metrics, while others, such as @Thomas1998 and @Taylor2011, argued that a correlation of $0.7$ is sufficiently high.

We try to provide clarity in this debate by considering four important points:

1.  The agreement between metrics and peer review depends on the level of aggregation. The level of individual publications constitutes the lowest level of aggregation. The level of researchers and the level of research institutions represent higher levels of aggregation.

2.  At aggregate levels, metrics and peer review may take a size-dependent perspective—scaling with the size of an institution—or a size-independent 