By doing a little digging I found the source Oxfam report:
https://oxfamilibrary.openrepository...-210119-en.pdf (.PDF).
And I found their notes on methodolgy:
https://oxfamilibrary.openrepository...-210119-en.pdf (.PDF).
Most of the 28 pages on methodology concern analyzing billionaires. What I'm looking for is data to support the claim "3.8 billion of the world's poorest got 11% poorer." And I found it:
In short, they're guessing: "however, as no country has a single comprehensive source of information on personal wealth, and some others have few records of any kind, different methods are employed to estimate wealth figures when missing. As a result, wealth estimates show different quality levels."
So I went to thier source, Credit Suisse:
https://www.credit-suisse.com/media/...rt-2018-en.pdf (.PDF). After extensive search through the document, I can find no supporting data there. Basically, what I see is a report that says wealth increased at a rate of 4.6% and that the percentage of wealth at the top 1% grew faster than that of the rest. I don't see any claim the lowest 50% lost anything, they just have a lower percent--but you can't trnslate that into dollars and say they lost something. And, as already pointed out, even the Credit Suisse data is guesswork without a solid foundation of data.
The entire purpose of the Oxfam report is stated succinctly on the title page: "Universal health, education and other public services reduce the gap between rich and poor, and between women and men. Fairer taxation of the wealthiest can help pay for them. " In short, redistribution.
The old socialist argument was the workers are exploited. The workers abandoned them to bargain with the capitalist for better wages, benefits, conditions, etc. So socialist now turn to the income inequality argument. That argument is losing out to the face the global poverty rate has been in sharp decline.
Anyway, there's the source of data. I open to any factual disagreement. Do your homework.