Although from a US perspective, so some elements need translation, this article on how the quality of firms should be measured provides a valuable challenge to traditional assumptions about size, profitability and history as proxies for quality of advice.
The most important point is the one quoted below. Nothing a firm says about itself, nor any of the metrics (such as PPP) that describe the firm without external referents, should be taken seriously. The only measure of quality is how successfully the work it does meets clients' needs. And clients are the only ones who can define 'need' and 'success' in this context.
So what are some better ways to measure law firm quality? A good place to start is from the client perspective. Some obvious criteria are: results; areas of law firm excellence; client business comprehension (think: competency based testing); client retention; attorney retention; firm succession plan; collaboration with clients and others in the legal supply chain; billing flexibility and alignment of financial interest with clients; diversity; innovation; and pro bono/community involvement.