ITEM 4.4.b. OF THE AGENDA TYPOLOGY OF SAMPLING STRATEGIES UPDATED METHODOLOGICAL INFORMATION. Doc. ESTAT/F5/ES/201 (available in EN) Orig.

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1 Luxembourg, May 2010 AC/RDR/AMP Drectorate F: Socal Statstcs and Informaton Socety Unt F5: Health and food safety; Crme Doc. (avalable n EN) Org. : EN Workng group Food Safety Statstcs" Meetng of June 2010 Luxembourg Char: Mrs. A. CLEMENCEAU ITEM 4.4.b. OF THE AGENDA UPDATED METHODOLOGICAL INFORMATION TYPOLOGY OF SAMPLING STRATEGIES

2 Eurostat/Food safety statstcs Workng group Food safety statstcs, June 2010 Typology of samplng strateges Summary The partcpants to the Task Force "Control and montorng actvtes" set up n 2004, have always rased the mportance of a common glossary as a basc tool to compare not only data among countres but also among dfferent control areas n the same country. Ths s currently dffcult due to the specfc termnology used n each area. As a consequence, one of the frst tasks carred out by the Task Force was the creaton of a common glossary. Eurostat proposed a pragmatc approach 1, based on fve prncples, for buldng the glossary: ncluson only of terms necessary to nterpret clearly the data collected on control and montorng actvtes (the defntons must be compatble wth those present n EU legslaton); excluson of very specfc and techncal terms used exclusvely n a partcular control doman; ncluson of the concept "samplng strategy" and the typology of "samplng strateges". At the moment, each control/montorng doman uses a specfc termnology: Actve montorng, passve survellance, follow-up samplng, renforcement samplng, routne montorng, suspect samples and target samples are examples of terms used currently n the dfferent control domans to descrbe the strategy and to report the results. In other domans ths nformaton s not even provded. The objectve of ths document s that of nformng on the harmonsed and smplfed way to document whch strategy s used to run the control and montorng actvtes n the dfferent domans reported to the EU and ncluded n the controls database, lke montorng of zoonoses, montorng of pestcde resdues, offcal food and feed controls, TSE montorng, etc. The fnal agreed proposal s part of the fnal verson of the common glossary. It s also ntegrated n the last verson of the controls database n order to contrbute to a better nterpretaton of the data both for the actvty of "nspectons" and of "samplng for analyss". 1 See the pragmatc approach proposed by Eurostat n Doc. ESTAT/D6/ES/104: Common glossary of control and montorng actvtes.

3 Typology of samplng strateges Objectve... 1 Hstory of the process... 1 Samplng strategy Concept defnton... 2 Samplng strategy s typology... 3 Samplng strateges: Defntons and gudelnes... 3 Objectve samplng... 3 Selectve samplng... 4 Census... 7 Suspect samplng... 7 Convenent samplng... 8 Other samplng strateges Concluson...8 Objectve The dentfcaton of the strategy used by the countres to select the sample of the target populaton that wll be subject to controls s a key ssue to help n the correct nterpretaton of the results obtaned from those controls. At the moment, each control/montorng doman uses a specfc termnology: Actve montorng, passve survellance, follow-up samplng, renforcement samplng, routne montorng, suspect samples and target samples are examples of terms used currently n the dfferent control domans to descrbe the strategy and to report the results. In other domans ths nformaton s not even provded. The objectve of ths document s that of nformng on the harmonsed and smplfed way to document whch strategy s used to run the control and montorng actvtes n the dfferent domans reported to the EU and ncluded n a sngle controls database, lke montorng of zoonoses, montorng of pestcde resdues, offcal food and feed controls, TSE montorng, etc. Hstory of the process The dentfcaton of the man strateges used n control and montorng actvtes has proved to be a dffcult task. A frst proposal (of November 2005) dentfed and defned 3 man types of samplng strateges: objectve samplng, selectve samplng and suspect samplng 2. It was ncluded n the Commsson "Gudance document on offcal controls, under regulaton (EC) no 882/2004, concernng mcrobologcal samplng and testng of foodstuffs" 3. Ths typology s also proposed by the European Food Safety Authorty (EFSA) n the manual for the annual reportng on the montorng of zoonoses. The typology was revsed by the Task Force Control and montorng actvtes n 2006 as some countres found dffcultes when tryng to apply t at natonal level. The second proposal (of May 2006) whch was consdered better for data evaluaton, dentfed 4 man types of samplng strateges resultng by the combnaton of the choce of the target populaton (rsk/non-rsk based) and the samplng method 4. However, the new proposal looked more 2 See Doc. ESTAT/D6/ES/104 Rev.1 3 Avalable at: 4 These 4 strateges were the followng: Type 1 (rsk based populaton selecton and random samplng), Type 2 (Rsk based populaton selecton and non-random samplng), Type 3 (no rsk based populaton selecton and random samplng) and Type 4 (no rsk based populaton selecton and non random samplng). 1

4 complex and the Task Force recommended Eurostat to provde more detaled defntons and gudelnes to help the countres to dentfy the samplng strategy type appled n each case. As stll dffcultes remaned for some Member States, Eurostat decded to revew the whole approach tryng to propose a typology that could be applcable at natonal level. At meetng of the Techncal Group Food and feed control and montorng actvtes held on 1 2 October 2007, Eurostat presented a proposal ntended to be an mprovement of the ntal "3-types" typology, focusng exclusvely on the crtera used to select the unts that wll be controlled. After the fnal agreement, Member States were asked, on the bass of a Questonnare prepared by Eurostat wth the proposal for the correspondence between current termnology and typology of samplng strateges, to check whether the samplng strateges assgned for each doman were approprate for ther countres and, f necessary, correct them 5. At the meetng of Techncal Group Food and feed control and montorng actvtes held on March 2009, Eurostat presented the results of the reples to the Questonnare provded by the countres on the samplng strateges used 6. At the same tme Eurostat provded further clarfcaton n order to help the countres to assgn correctly the samplng strateges n the dfferent controls domans and nvted the mssng countres to send the reples by the end of Aprl The nformatons provded have been ncluded n the Controls database, n order to nterpret the data. Durng 2009 Eurostat has collaborated wth the EFSA Workng Group on Data Interchange" as promotng the coordnaton between SANCO, EFSA and Eurostat and workng on data collected by SANCO and EFSA. In ths context, the defntons of typologes of samplng strateges have been further defned and they were ncluded n the EFSA document Standard sample descrpton for food and feed 8. Ths document s concerned wth ths fnal verson of the samplng strateges. Samplng strategy Concept defnton The "samplng strategy" can be defned as the approach used to select the unts of the target populaton 9 subject to controls: busnesses, anmals, foodstuffs, etc. It s worth notng that the comparablty and nterpretaton of the results reles on the samplng strategy but as well on other parameters lke the analyss methods, analyss matrces, preparaton of samples, methods of calculaton of the results, etc. 5 See Doc. ESTAT/F5/ES/104 Rev.3, Part 1 Typology of samplng strateges presented at the meetng of Techncal Group Food and feed control and montorng actvtes on 1 2 October 2007, and Doc. ESTAT/F5/ES/104 Rev.4 Typology of samplng strateges - Verson of February The Questonnare n the Annex: Proposal for the correspondence table between current termnology and typology of samplng strateges are flled dfferently n the two documents, as the Doc. ESTAT/F5/ES/104 Rev.4 s a revson of the prevous "Doc.ESTAT/F5/ES/104 Rev.3. Part 1" followng a further n-depth analyss and comments receved durng and after the meetng of the Techncal Group Food and feed control and montorng actvtes of 1 2 October See Doc. ESTAT/F5/ES/177 Rev.1 Results of the Questonnare to ndcate the samplng strateges used presented at the meetng of the Techncal Group Food and feed control and montorng actvtes on March See Doc. ESTAT/F5/ES/197 Rev.1 Fnal mnutes of meetng of the Techncal Group Food and feed control and montorng actvtes on March Avalable at: 9 Target populaton refers to the populaton that s the object of study. 2

5 The samplng strategy s generally decded at a hgh level n the natonal herarchy and mght be dfferent dependng on the control doman. The practcal mplementaton of the strategy s carred out locally by nspectors whch mght take ther own decsons regardng, for example, when to vst the establshment and what to sample wthn that establshment. Samplng strategy s typology The object of the control/montorng actvty, the busnesses, food products, lve anmals, etc. s the "target populaton". Ths s the populaton from whch the control/montorng actvtes could reach conclusons. The target populaton can be rsk based determned or not. The potental rsk can be dentfed on a scentfc bass (results of scentfc studes, knowledge of current producton practces, etc.) or on the bass of a suspcon (nformaton exchanged wthn the Rapd Alert System for Food and Feed, prevous non-conformty results for the same product/ establshment). Once the target populaton s dentfed, a samplng method s used to select the sample of the "target populaton" that wll be controlled/montored n realty. The samplng method used to select the sample can be ether "random" 10, provdng results from whch conclusons can be appled to the "target populaton", or "non-random". A frst key ssue s to dentfy clearly the target populaton, whch can be ether the establshments of the food chan (.e. nspectons of hygene condtons of retalers, HACCP controls, etc.), or specfc products (.e. montorng of pestcde resdues, controls for the presence of undesrable substances n anmal nutrton, etc). In the second case, the selecton of the products wll be done n many cases by selectng the establshments where they wll be controlled/montored. Besdes, the correct dentfcaton of a samplng strategy depends on the reportng of the data, as shown n the next paragraph. From the analyss of the nformaton already provded to the European Commsson and to EFSA, t appears that there are fve man types of samplng strateges that are currently used n the control domans: Objectve samplng, Selectve samplng, Suspect samplng, Convenent samplng and Census. Samplng strateges: Defntons and gudelnes 11 Samplng strategy descrbes how the sample 12 was selected from the populaton beng montored or surveyed. The defntons refer to reportng of aggregate data. Objectve samplng Defnton: strategy based on the selecton of a random sample 13 from a populaton on whch the data are reported. 10 The method used to select the samples s any method (pure random, systematc, stratfed, cluster samplng) leadng to results applcable to the dentfed target populaton. 11 The gudelnes are not ntended to recommend the use of a certan strategy but just to help the countres to dentfy the strategy/strateges currently used, wth the objectve of documentng properly the data on control and montorng actvtes. 12 A sample s a subset of a populaton, see defnton n Doc. ESTAT/F5/ES/ Random sample s a sample n whch each unt of the populaton has the same probablty to be ncluded. A random sample allows mplementng statstcal nference, that s, t leads to conclusons on the target populaton. A more general term for a random sample s probablstc sample : each populaton unt has a postve and known probablty of beng selected to sample. 3

6 Gudelnes: The method used to drawn the sample (establshments or commodtes) could be any method (smple random, systematc, stratfed, cluster samplng, multstage samplng) leadng to results representatve of the determned target populaton. In partcular, t ncludes also those cases where the target populaton s stratfed n subpopulaton and the samplng s run wth proportonal crteron. Ths strategy provdes data enablng the realsaton of statstcal nference. Ths strategy s one of those generally used for planned montorng programmes ntended to provde an overvew of the stuaton regardng a certan food safety aspect. Examples: random selecton of bovnes older than 24 months n all certfed slaughterhouses, for checks of TSE. (Purpose: To know the overall stuaton regardng the presence of BSE n bovnes over 24 months slaughtered for human consumpton.) stratfed regonal selecton of food producers for HACCP controls. It would gve results that could be expanded to all the populaton and breakdown by regon. The results must be weghted accordng to the number of food producers of each regon. (Purpose: to know the overall stuaton regardng HACCP complance of food producers. Results are provded for "all food producers". It would also be possble to provde results at regonal level.) controls for the presence of Lstera monocytogenes n cheeses obtaned from raw mlk, from a random selecton of producers of raw mlk cheeses; (Purpose: to know the overall stuaton regardng presence of Lstera monocytogenes n cheeses produced from raw mlk. The target populaton s "Raw mlk cheese". Results are provded for raw mlk cheeses.) controls for the presence of Lstera monocytogenes n cheese from a stratfed selecton of cheese producers, grouped by the relatve mportance of ther producton of raw mlk cheese compared to pasteursed mlk cheese. The results must be weghted to take nto account each type of cheese producers and ther cheese producton (Purpose: To know the overall stuaton regardng presence of Lstera monocytogenes n all cheeses. Target populaton s "Cheese". Results are provded for cheese. They could also be broken down for each of the 2 cheese types and for both the cheese type and the level of producton.) controls for the presence of Lstera monocytogenes n cheeses obtaned from pasteursed mlk, from a regonally stratfed selecton of pasteursed mlk producers. (Purpose: to know the overall stuaton regardng presence of Lstera monocytogenes n cheeses produced from pasteursed mlk. The target populaton s "Pasteursed mlk cheese". Results are provded for pasteursed mlk cheeses. It would also be possble to provde results at regonal level.) Selectve samplng Defnton: strategy based on the selecton of a random sample from a subpopulaton (or more frequently from subpopulatons) of a populaton on whch the data are reported. The subpopulatons are determned on a rsk bass or not. The samplng from each subpopulaton s not proportonal: the sample sze s proportonally bgger for nstance n subpopulatons consdered at hgh rsk. Ths samplng ncludes also the case when the data reported refer to censuses on subpopulatons. Gudelnes: The samplng s delberately based, wth respect the overall populaton on whch the data are reported, and s drected at partcular products or manufacturers. The crtera used to select the subpopulaton from whch to draw the sample s based on a scentfc bass (results of scentfc studes, knowledge of current producton practces, etc.) or on the bass of prevous 4

7 nformaton (non-conformty results for the same products/ establshments for prevous years, nformaton exchanged wthn the Rapd Alert System). Ths strategy provdes data whch does not enable the realsaton of statstcal nference on the populaton that s the object of the control actvty. It only provdes nformaton on the subpopulatons. If a subpopulaton s determned on a rsk bass, t s worth notng that even though several countres use the same type of samplng strategy, the results would not be necessarly comparable as the defnton of rsk mght be dfferent n each country. Practcally ths would mean that these countres determne dfferent subpopulatons. Besdes, the dfferent montorng systems address rsk n dfferent forms. Ths strategy s one of those used for planned montorng and control programmes ntended to reduce non-conformtes. Examples: controls for doxns n feed materals n agrcultural holdngs, where sample unts are taken preferably from establshments close to potentally pollutant enterprses; (Purpose: to prevent the presence of doxns n feed materals. Results are provded for all agrcultural holdngs wth hgh rsk of presence of doxns.) checks for the presence of unauthorsed veternary drugs n randomly selected bovnes enterng the slaughterhouses from farms where postve results were found the prevous year; (Purpose: to detect the use of unauthorsed veternary drugs n bovnes. Results are provded for hgh rsk farms.) at the slaughterhouses, selecton of anmals showng sgns of njecton for checkng presence of unauthorsed veternary drugs; (Purpose: to detect the use of unauthorsed veternary drugs. Results are provded for suspected anmals.) controls for BSE n some anmals showng sgns at the ante-mortem nspectons. (Purpose: to prevent BSE anmals enterng the food producton chan. Results are provded for bovnes showng sgns at the ante-mortem.) How to consder the results of a stratfed samplng? Objectve or Selectve samplng? In stratfed samplng the target populaton s splt n 2 or more subpopulatons ( strata ) accordng to some crteron. If the crteron s the level of rsk, we can magne for nstance to splt the populaton n 4 subpopulatons accordng 4 levels of rsk, from the lowest to the hghest. Then a sample s drawn from each subpopulaton, the results of the number of non-complyng sample unts n each sample are smply summed up and the total obtaned s reported as number of non-complyng sample unts n the overall sample. Is ths samplng Objectve or Selectve? In other words, does the result on the overall sample, calculated as above, provde conclusons that can be appled to the "target populaton"? It depends on the sample sze of each stratum and on the varablty nsde each subpopulaton. Let s consder only the sample sze. a) If the sample sze s bgger for subpopulatons consdered hgh rsk than for the ones consdered low rsk, the global effect on the estmate (total or percentage) of non complance n the populaton could be unbased towards hgh rsk subpopulaton; n other words, the overall level of non complance s overestmated. In ths case the samplng should be classfed as Selectve, as the result reflects more the stuaton n hgh rsk subpopulatons. 5

8 b) If the total sample s allocated among the strata usng the Proportonal allocaton crteron (that s: the samplng rates f = n N are constant n all the subpopulatons) the global effect on the estmate of non complance of the populaton should be unbased, and the samplng can be classfed as Objectve. Let s consder the mplcatons on the estmaton of a percentage of non-complyng sample unts related to the overall populaton. An unbased 14 estmate of such percentage s calculated by summng up the weghed percentages of non-complance n each sample as follows: m m ) ) N x p = p = 1 N = 1 n N = N where: p ) s the estmate of percentage of non-complance n the populaton, m s the number of subpopulatons ( = 1, m), p ) s the percentage of non-complance n the sample drawn from the subpopulaton, N s the sze of the subpopulaton, N s the sze of the populaton, n s the sze of the sample drawn from the subpopulaton, x s the number of non-complyng sample unts n the sample drawn from the subpopulaton, n s the sze of the overall sample, x s the number non-complyng sample unts n the overall sample. In practce, n controls and montorng actvtes, the data collected from dfferent subpopulatons are very often summed up wthout beng weghted. Then the percentage of non complance s calculated as follows: n ) x j x p = = n n =1 where x j may assume value 1 (non-complyng sample unt) or 0 (complyng sample unt) and x s the total number of non-complyng sample unts n the sample. Is ths percentage an unbased estmate of percentage of non-complance n the populaton? Yes, f the total sample s allocated among the strata usng the Proportonal allocaton, n fact: n n f = = = f, N N t follows that n N = n N and 14 In statstcs, bas of an estmator s the dfference between ths estmator's expected value and the true value of the parameter beng estmated. An estmator wth zero bas s called unbased. Otherwse the estmator s sad to be based. 6

9 m m m ) N x n x p = ) p = = = 1 N = 1 n n = 1 n = x n As a general rule, the results obtaned from the sample drawn by a method that mples a subdvson of the target populaton, say stratfcaton or clusterng, should be weghted accordng to that subdvson n order to provde representatve results of the whole target populaton (Objectve samplng). If ths s not done, the results should be reported separately for each subpopulaton group. If the results by strata or cluster are summed up wthout weghtng, ths should be clearly mentoned as no concluson can be drawn from them (Selectve samplng). Census When the totalty of a populaton, on whch the data are reported, s controlled. Gudelnes: followng the defnton, f a census of a subpopulaton of a populaton s ncluded n the reportng referred to the populaton, the samplng referred to the populaton s Selectve. For example, data on controls on mported grapes ncludng a census of grapes mported from Italy and samplng on grapes from other countres, should be reported as follows: Grapes: Selectve samplng, Grapes from Italy: Census. Example Hygene nspectons of all regstered slaughterhouses (Purpose: to know the overall stuaton of regstered slaughterhouses regardng complance wth hygene legslaton). BSE checks of all bovnes older than 30 months n all certfed slaughterhouses. (Purpose: To know the overall stuaton regardng the presence of BSE n anmals slaughtered for human consumpton.) Suspect samplng Defnton: selecton of an ndvdual product or establshment n order to confrm or reject a suspcon of non-conformty. It's a not random samplng. The data reported refer themselves to suspect unts of the populaton. Gudelnes: The selecton s focused on a partcular product or manufacturer. The choce s based on the judgement and experence regardng that product or manufacturer. Ths strategy s ntended to reduce the non-conformtes. Results can not be generalsed and refer only to the ndvdual product or establshment. Examples: Controls to verfy a problem reported by a consumer n a product obtaned from a certan producer; (Purpose: to verfy f the problem remans n order to take the necessary actons. Results are provded for the concerned product and producer.) Checks for the presence of pestcde resdues n oranges avalable from a precse wholesaler, as a consequence of prevous non-conformty results; (Purpose: to verfy f non-conformty perssts or not. Results refer exclusvely to the precse wholesaler and frut). 7

10 Convenent samplng Defnton: Strategy based on the selecton of a sample for whch unts are selected only on the bass of feasblty or ease of data collecton. It's a not random samplng. The data reported refer themselves to unts selected accordng to ths strategy. Ths new typology was added as ncluded n samplng strateges used n data collected by EFSA. Other samplng strateges In order to document the Controls database, further typologes of samplng strateges can be assgned f none of the prevous ones has been used for the selecton of the sample unts: More than one samplng strategy: the data reported refer to sample unts selected accordng more than one samplng strategy, for example: some unts are selected accordng Objectve samplng and other unts accordng Suspect samplng. Other: the data reported refer to sample unts selected accordng a strategy not ncluded n the prevous ones. Not specfed: the data reported refer to sample unts selected accordng a strategy not specfed. Concluson The objectve of ths document s that of nformng on the harmonsed and smplfed way to document whch strategy s used to run the control and montorng actvtes n the dfferent domans reported to the EU. The documentaton of the samplng strateges used by countres n the dfferent domans s ncluded n the controls database n order to contrbute to a better nterpretaton of the data both for the actvty of "nspectons" and of "samplng for analyss". 8