The title of this blog post is a Latin legal phrase meaning “false in one thing, false in everything”. It refers to a principle about regarding everything a witness says as not credible, if one thing said by the witness is proven not to be true. This has been a part of the plot in plenty of courtroom films and TV-shows.
This principle has meaning related to data quality too. An example from direct marketing will be a receiver of a direct mail saying: “If you can’t get my name right, how can I trust you in getting anything right during a purchase?”
An example from the multi-channel world, or should we say omni-channel today, would be a shopper saying: “If you say one thing about the product in the shop and another thing on the website, how can I trust any of your product information?” Falsehood in omni-channel so to speak.
Measuring the impact of such attitudes and thereby the Return on Investment (ROI) in data quality improvement based on this principle is very hard. We usually only have random anecdotal evidence about that this happens.
But, what we can say is: Don’t lie in court and don’t neglect your data quality. It will hurt your credibility and then in the end your creditworthiness.
Liliendahl on Data Quality 
While I understand why the legal principal exists, in the non legal world, there is a lot of subtlety. Even a crook lying on the witness stand may actually be telling the truth in other areas of testimony. For data quality, this translates to: you may trust one source in one domain, but not so much for another domain. The real trick is to figure out which source is the best in a particular situation. For any particular transaction, the best sources of master data may span both inside and outside an organizations virtual walls.
In South African we have seen our financial sector knocked down by the failings of a single bank – tarred with the same brush – however unfairly.
My post http://blog.masterdata.co.za/2014/09/11/compliance-data-governance-to-restore-trust/ discusses how trust can be regained.
It is a real world example of the principle you have described here.
Thanks Gino and Gary for adding in.
The saying: “For any particular transaction, the best sources of master data may span both inside and outside an organizations virtual walls” resonates very well with by approach to data governance, data quality and MDM.