What makes statistics economical reports so varied and different?

Question

Search for `Iran's GDP` in Google, and what you will see is an image representing GDP of Iran, compared to other neighbor countries. It gives us the value of **393.4 billion USD (2015)**.Now in that page simply open some results. From Wikipedia, from World Economics, and other sources. You see values from 300 to 1.3 trillion Dollars.Values are not coherent with each other. As an engineer who is not an economist but understands figures and diagrams, I think these values are so varied that they can not be used in researches as reliable information.I'm working on a proposal, and I need information that can be trusted. What makes such a bold and universally known economical value so varied across data sources?

Answer 1

In economics, business and many other areas sources are very important. Many sources have agendas in publishing certain information, so only use sources that are validated. For example, for GDP, I would use the trusted International Economic data and Statistics such as International Financial Statistics, World Bank Data, and Bloomberg. Take a look at Yale's law library list of 75 sources per the following link of economic data, statistics, reports, and commentary. I hope that this helps!

https://library.law.yale.edu/news/75-sources-economic-data-statistics-reports-and-commentary

Answer 2

I think you may be confusing estimates of the real value of Iran's GDP and it's nominal value. According to here (http://www.payvand.com/news/10/apr/1042.html), the nominal GDP is projected to double in 5 years. I recommend paying careful attention to whether figures are real or nominal GDP and to what base year the real GDP is tied. The World Bank provides pretty good estimates of Iran's GDP, but the valuation of its oil and natural gas reserves and the effects of sanctions create uncertainty.

Also, most of the world doesn't collect as detailed statistics as the US does and so it's often necessary to use fallible short-cuts and that can result in varying estimates of GDP with contraband being harder to measure accurately. There are also price controls in Iran and so it may be hard to measure the effective prices of many goods and the rate of inflation.