Confidence Interval - When to use Standard Error vs Standard deviation

CFA Level I Quant
#1

Hello all,

I’ve been reviewing quant and I can’t seem to get this right. When do we use the standard error vs standard deviation when calculating confidence intervals.

That is Xbar ± Se * Critical value vs Xbar += Sd*critical value.

Another question regarding this, is when there are confidence intervals based purely off critical values vs off the SE*Crit;SD*Crit.

So have been reading financial exam help, and there is a differentiation there. Are we only using the SE when we are estimating where the population mean will ie? If so, then how does the above work with Hypothesis testing?

Edit: This is making sense now. Use SD when we want to know where the mean will lie, 90% range.

Thank you!

*I searched and there was talk about knowing a population SD and what not, however that didn’t prove true in my readings.

#2

You use standard deviation when you’re creating a confidence interval for a random (arbitrary) value from the population.

You use standard error when you’re creating a confidence interval for the mean of a population.