FBI Table 5: Crime in the United States by State 
shows that South Dakota has very little robbery, just 24.7% of the National Average rate compared to Texas where the robbery rate is 114.1%. But their rates of violent crime, murder, rape and aggravated assault are all around the National Average, as in Texas. So
why does South Dakota have far fewer robberies - just 21.7% of the rate in Texas?
The Study by the Crime Prevention Research Center Table 3 
shows the cost of a handgun permit as $10 in South Dakota while the cost in Texas is substantially higher at $140. Their table 2 emphasizes the result of discouraging citizens from protecting themselves and their family: 14.7% of adults in South Dakota have concealed carry permits while far fewer adults - just 5.4% - have one in Texas.
They also found that Texas with over twice the population of Pennsylvania has 1.15 million permit holders to Pennsylvania's 1.28 million. It's not that Texans don't like guns - it's simply that they have a more difficult time getting permits. The fee for a 5-year permit is only $20 in Pennsylvania.
Clearly there are other factors as well as demographics and population producing such dramatically lower robbery. But since the other crime rates are similar if not a little higher,
why just robbery? The FBI data verifies the Concealed Carry study findings:
the more you encourage armed citizens, the more you discourage armed robbery!
A study shows that
South Dakota is the best state for middle-class individuals to live 
. Could their very low robberies caused by their widespread gun ownership have anything to do with South Dakota's top-place ranking?
Update.
The latest 2019 full year crime figures from the FBI show a similar result. The South Dakota 2019 violent crime rate at 399.0 per 100,000 inhabitants is more or less the same as the 418.9 in Texas, yet the SD robbery rate is 22.0 per 100,000 while TX is 100.0 - over four times higher, not much different to the 2015 comparison in the diagram.
In 2017,
Texas passed a bill to lower the cost of a permit somewhat to $40, but NOT to SD's $10. An improvement, yet their politicians could do far more to further encourage law-abiding citizens to protect themselves, their family and their property from violent criminals.