To round numbers to the nearest thousandths, you have to pretend that you're using a number grid, starting with 0, where numbers are 0.001 units apart, and there's nothing in between.
So, the grid looks like
[tex] \ldots, -0.003, -0.002, -0.001, 0, 0.001, 0.002, 0.003, \ldots [/tex]
Now the two numbers in this grid which surround 0.9836 are 0.983 and 0.984, and we have to choose the closest one. Since
[tex] |0.9836-0.983| = 0.0006,\text{ and } |0.9836-0.984| = 0.0004 [/tex]
we can see that 0.984 is less distant from 0.9836, and so we can round 0.9836 to 0.984
