Context : The classification of young stellar objects ( YSOs ) is typically done using the infrared spectral slope or bolometric temperature , but either can result in contamination of samples .
More accurate methods to determine the evolutionary stage of YSOs will improve the reliability of statistics for the embedded YSO population and provide more robust stage lifetimes .

Aims : We aim to separate the truly embedded YSOs from more evolved sources .

Methods : Maps of HCO ^ { + } J =4–3 and C ^ { 18 } O J =3–2 were observed with HARP on the James Clerk Maxwell Telescope ( JCMT ) for a sample of 56 candidate YSOs in Perseus and Taurus in order to characterize the presence and morphology of emission from high density ( n _ { \mathrm { crit } } ¿10 ^ { 6 } cm ^ { -3 } ) and high column density gas , respectively .
These are supplemented with archival dust continuum maps observed with SCUBA on the JCMT and Herschel PACS to compare the morphology of the gas and dust in the protostellar envelopes .
The spatial concentration of HCO ^ { + } J =4–3 and 850 \mu m dust emission are used to classify the embedded nature of YSOs .

Results : Approximately 30 % of Class 0+I sources in Perseus and Taurus are not Stage I , but are likely to be more evolved Stage II pre-main sequence ( PMS ) stars with disks .
An additional 16 % are confused sources with an uncertain evolutionary stage .
Outflows are found to make a negligible contribution to the integrated HCO ^ { + } intensity for the majority of sources in this study .

Conclusions : Separating classifications by cloud reveals that a high percentage of the Class 0+I sources in the Perseus star forming region are truly embedded Stage I sources ( 71 % ) , while the Taurus cloud hosts a majority of evolved PMS stars with disks ( 68 % ) .
The concentration factor method is useful to correct misidentified embedded YSOs , yielding higher accuracy for YSO population statistics and Stage timescales .
Current estimates ( 0.54 Myr ) may overpredict the Stage I lifetime on the order of 30 % , resulting in timescales of 0.38 Myr for the embedded phase .