ACES proficiency tests
Stockholm University
106 91 Stockholm

ACES website:


History of the program

The Swedish EPA sponsored the first proficiency test for environmental laboratories in 1973. SEPA's laboratories were transferred to Stockholm University in 1992 forming part of the Institute for Applied Environmental Research (ITM) which took over the commission. In 2015 ITM, analytical chemistry and environmental chemistry merged to form the Department of Environmental Science and Analytical Chemistry (ACES).


Participation in proficiency tests is a crucial part of a quality control system and mandatory for accredited laboratories. By performing analyses on the same samples it is possible to compare results, enabling the identification of inadequate routines and methods. Accrediation bodies use results from proficiency tests when auditing accredited laboratories.


Participation is confidential. Laboratories are assigned a lab code for identification in the reports.

If there are few participants

When registering it is not generally necessary to specify which parameters your lab wishes analyze. Certain parameters (including a number of metals) often have very few participants. For these we request advance notice. If too few labs show interest we will cancel the parameter. In this case, registered labs will be contacted before the samples are sent out.
Results for every parameter included in the test round will be published in the report. See "Criteria for the calculation of z-scores" for how we evaluate results when participation is low.

Criteria for the calculation of z-scores

Parameters with at least six acceptable results: z-scores are calculated if the samples are homogenous and stabile with respect to that parameter.

Parameters with four or five acceptable results, and no more than one outlier: z-scores are calculated if the CV% (standard deviation/mean) is less than 10%. Less-than values and obvious clerical errors are not counted as outliers.

Parameters that do not fulfil the above conditions: no z-scores are calculated. Only the results of the participants are presented. Basic statistics and the Youden plot (without the confidence interval) may be included depending on the number of participants and the distribution.

Sample pairs and the Youden technique

Each proficiency test (part) consists of a pair of samples. The water for each of the two samples have the same source but are mixed in separate vats. Consquently the results of the two samples should differ somewhat. This allows us to use the Youden technique to reveal systematic errors.

In the Youden plot the results of each sample pair (sample 1—sample 2) are plotted in a coordinate system. Systematic and random errors are revealed by the scattering of the results. Results distributed along the 45 axis indicate the presence of systematic errors. A circular distribution pattern is indicative of randomly distributed results.

About us

ACES is charged with conducting proficiency tests in natural and wastewaters by SWEDAC (Swedish Board for Accreditation and Conformity Assessment). The annual program includes rounds for metals and trace elements, ionic balance, phosphorous and nitrogen salts, wastewater parameters and suspended solids. Chlorophyll, CrVI and extreme pH are included every few years. Typical matrices are treated outgoing municipal and industrial wastewaters, and potable and recipient water bodies, all with levels representative for Scandinavian conditions.