You need more than one measurement in order to calculate a standard deviation...  Highlight more than one record and then display the statistics plot.

It is always recommended to perform at least 3 measurements or more for each sample for DLS measurements which will then clearly give an idea of what you are measuring and what values you are expecting from it. You can then calculate standard deviation (SD) after that. 

Thanks

It is not directly clear what you consider unreasonable.

• If it is the mean of your obtained distribution, then possibly your sample is aggregated.
• If it is the width of your obtained distribution, then keep in mind that DLS is an inherently low resolution technique.

The best way to determine statistical significance of your result would be repeated measurements. Please also see "Peak size or mean - which one to pick in DLS" at http://www.materials-talks.com/blog/2014/07/10/faq-peak-size-or-z-average-size-which-one-to-pick-in-dls/

and answers to a similar question "How is the standard deviation calculated by the Zetasizer software?" at https://www.researchgate.net/post/How_standard_deviation_is_calculated_by_DLS_software

It is ok. But i need to know the sample that you used and the number of measurements

Polydispersity