Statistische of niet-statistische steekproeven

Presentatie over: "Statistische of niet-statistische steekproeven"— Transcript van de presentatie:

1 Statistische of niet-statistische steekproeven
G.B. Broeze P.C. Waas 10 juni Symposium Statistical Auditing

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Onderwerpen wel of geen detailwaarnemingen statistische of niet-statistische steekproef selectiemethoden 10 juni Symposium Statistical Auditing

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Inleiding ISA 200 Overall Objectives of the independent Auditor and the conduct of an audit in accordance with International Standards on Auditing 5. As the basis for the auditor’s opinion, ISAs require the auditor to obtain reasonable assurance about whether the financial statements as a whole are free from material misstatement, whether due to fraud or error. … 10 juni Symposium Statistical Auditing

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Detailwaarnemingen? ISA 330 The Auditor’s responses to assessed risks 18. Irrespective of the assessed risks of material misstatement, the auditor shall design and perform substantive procedures for each material class of transactions, account balance, and disclosure. A42 … This requirement reflects the facts that: the auditor’s assessment of risk is judgmental and so may not identify all risks of material misstatement; and (b) there are inherent limitations to internal control, including management override. 10 juni Symposium Statistical Auditing

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Detailwaarnemingen? Unless an audit organisation has sufficient evidence on the validity of its risk assessment, it should assume that this risk assessment is not valid in the sense that it may replace substantive audit. Broeze, G.H. (2006): Validation of Risk Assessment in Auditing 10 juni Symposium Statistical Auditing

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Detailwaarnemingen? anything that can go wrong will go wrong (Murphy’s Law) factor subjective judgments into objective equations Bayesian Risk Analysis 10 juni Symposium Statistical Auditing

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Voorbeeld groothandel in computerapparatuur en computeronderdelen levering aan ondernemingen in andere lidstaten 'af magazijn‘ deze afhaaltransacties vertegenwoordigen een groot belang (30-50%); de winstmarge op de afhaaltransacties was klein goedkeurende verklaringen terecht?! ten onrechte vertrouwd op mededeling gecontroleerde en aanwezigheid van het vereiste formulier conclusie: geen deugdelijke grondslag Raad van Tucht, JT 10 juni Symposium Statistical Auditing

8 Statistische steekproef?
ISA 500 Audit evidence A52. … The means available to the auditor for selecting items for testing are: (a) Selecting all items (100% examination); (b) Selecting specific items; and (c) Audit sampling. A55. … selective examination of specific items does not provide audit evidence concerning the remainder of the population. A56. Audit sampling is designed to enable conclusions to be drawn about an entire population on the basis of testing a sample drawn from it. 10 juni Symposium Statistical Auditing

9 Statistische steekproef
ISA 530 Statistical Sampling A12. With statistical sampling, sample items are selected in a way that each sampling unit has a known probability of being selected. With non-statistical sampling, judgment is used to select sample items. Because the purpose of sampling is to provide a reasonable basis for the auditor to draw conclusions about the population from which the sample is selected, it is important that the auditor selects a representative sample, so that bias is avoided, by choosing sample items which have characteristics typical of the population. 10 juni Symposium Statistical Auditing

10 Statistische steekproef?
steekproeven vaak niet-statistisch (85%) vaak statistisch geëvalueerd (36%) vaak onjuist geëvalueerd (21%) geen maatregelen om “bias” te voorkomen Hunton, J.E. (2002): Sampling practices of auditors in public accounting, industry, and government. 10 juni Symposium Statistical Auditing

11 Statistische steekproef
Hall et al. investigate whether doubling the sample size mitigates the selection bias in haphazard sampling that is (or may be) caused by ‘call properties’ of units, as size or isolation (the larger and/or isolated are more likely to be selected). The bias clearly exists (some 30% over-representation), the doubling hardly has any effect. Hall, Thomas W.,Terri L. Herron, Bethane Jo Pierce and Terry J. Witt (2001): The Effectiveness of Increasing Sample Size to Mitigate the Influence of Population Characteristics in Haphazard Sampling. 10 juni Symposium Statistical Auditing

12 Statistische steekproef
In an experiment is found: Auditors tend to underestimate the effect of both projected error and uncertainty when evaluating aggregate error and the need for adjustments to financial statements. They even misjudge the meaning of an upper error bound (statistically derived) exceeding the materiality. Burgstahler, D., S.M Glover & J. Jiambalvo(2000):Error Projection and Uncertainty in the Evaluation of Aggregate Error Auditing: A Journal of Practice and Theory Vol. 19 No. 1 10 juni Symposium Statistical Auditing

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Voorbeeld grote bank niet bestaande miljoenenkredieten werden opgevoerd leningen op naam van zo’n zeshonderd kleine debiteuren die gezien de omvang van hun debet niet gecontroleerd werden fraude circa f 20 miljard interne en externe accountant merken dit niet op 10 juni Symposium Statistical Auditing

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Selectiemethode? ISA 530 Statistical Sampling random selection systematic selection monetary unit sampling haphazard selection block selection 10 juni Symposium Statistical Auditing

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Conclusie obtain reasonable assurance about whether the financial statements as a whole are free from material misstatement conclusions to be drawn about the entire population factor subjective judgments into objective equations (Bayesian Methods in Risk Assessment) capable of being tested (Popper, Karl R., 1959: The Logic of Scientific Discovery) 10 juni Symposium Statistical Auditing

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Vragen? 10 juni Symposium Statistical Auditing