Crj-200 flight crew reported that during descent they misunderstood instructions from ATC.

Attributes
ACN	1471455
Time
Date	201708
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Make Model Name	Regional Jet 200 ER/LR (CRJ200)
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Person 1
Function	First Officer Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Pilot Not Flying Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	ATC Issue All Types Deviation - Procedural Clearance Deviation - Speed All Types

Narrative:

We were descending through 13;000 ft on the arrival when ATC told us to descend to 11;000 ft and slow to 250. We began the descent and slowed to 250. ATC then asked us what our speed was; and we replied with 250 knots. The controller said we were told to descend to 11;000 ft then slow to 250 knots. Both the captain and I understood the clearance as descend and slow; not descend then slow. I believe the cause of this event was a very high workload due to thunderstorms ahead of us on the arrival; very busy airspace; and possible nonstandard phraseology. We should have questioned the controller; about the descent and slow down. Unfortunately both of us thought we understood the controller's instructions. From this point on we will make sure and ask the controller for clarification if there is any question with ATC instructions.

Original NASA ASRS Text

Title: CRJ-200 flight crew reported that during descent they misunderstood instructions from ATC.

Narrative: We were descending through 13;000 ft on the arrival when ATC told us to descend to 11;000 ft and slow to 250. We began the descent and slowed to 250. ATC then asked us what our speed was; and we replied with 250 knots. The controller said we were told to descend to 11;000 ft then slow to 250 knots. Both the Captain and I understood the clearance as descend and slow; not descend then slow. I believe the cause of this event was a very high workload due to thunderstorms ahead of us on the arrival; very busy airspace; and possible nonstandard phraseology. We should have questioned the controller; about the descent and slow down. Unfortunately both of us thought we understood the controller's instructions. From this point on we will make sure and ask the controller for clarification if there is any question with ATC instructions.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.