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Estimates and standard error of the estimates

The results published in the NFI tables are estimates for variables in the Swiss forest (population parameters) whose true values are not known and must therefore be extrapolated (estimated) from the NFI sample data.

The extrapolations carry uncertainties. However, the accuracy of the extrapolations can be reliably estimated from the NFI sample, which is designed to be a random sample. To this end, all NFI tables include a second figure in addition to the estimate itself, namely the standard error of the estimate.

In most tables the percent standard error is reported (±%), but the absolute standard error (±) is also reported occasionally, especially for estimated percentages. The relationship between absolute and percent standard error is as follows:

percent standard error = absolute standard error / estimate × 100

absolute standard error = percent standard error × estimate / 100

Information on the forest road network is based on a full survey of all forest roads. In this case, there is no need to specify a standard error because there is no uncertainty caused by sampling.

Confidence interval of the estimate

With the estimate and the standard error of the estimate, the limits of the so-called confidence interval of the estimate can be calculated as:

lower limit = estimate – tQ × absolute standard error

upper limit = estimate + tQ × absolute standard error

If the simple standard error is used for the calculation (tQ = 1), then the 68% confidence interval is formed. In this case, the true value of the population parameter has a 68% probability of lying within this confidence interval of the estimate. If the double standard error is used (tQ = 2), then there is a 95% probability that the true value lies within this 95% confidence interval.

Significance of the estimate

The confidence interval can be used to statistically test whether the estimated population parameter is larger or smaller than a specific reference or target value, or whether two estimated population parameters truly differ (in the real population). For practical applications, proceed as follows: if a reference value lies outside the confidence interval, the estimated population value can be interpreted as being significantly different from it; if it lies within the confidence interval, the difference between the estimated population parameter and the reference value can be interpreted as random or not significant. When two population parameters are compared, they can be interpreted as differing significantly if their confidence intervals do not overlap.

Dealing with missing values

When calculating a results table, data are not always available for all combinations of characteristics^* of the classification variables and regional demarcation applied. In most cases, this indicates that the attribute estimated with the respective target variable does not occur or occurs only very rarely. Usually, the value 0 is used (imputed). However, as this value is not based on direct measurements, the associated standard error is represented with a full stop [.]. If the imputed value of 0 is referred to in the calculation, e.g. in the case of percentages or certain change estimates, no value can be entered. In this case, the estimated value and standard error are both represented with a full stop [.].

For example, no Arolla pines have been found and measured in the Swiss Plateau to date (growing stock of Arolla pines by production region). The values can be assumed to be missing because Arolla pine does not occur in the Swiss Plateau and therefore the growing stock there must be 0.

^* e.g. ‘Arolla pine’ characteristic of the classification variable ‘tree species’ and ‘Swiss plateau’ characteristic of the ‘regional demarcation’

Changes

There are two types of changes in the NFI:

The first type involves specific target variables relating to components of change, such as increment, fellings, mortality and losses. These target variables are only available for two consecutive inventory cycles, e.g. NFI3–NFI4. In the evaluation of components of change, the classification variable characteristic of the second inventory cycle is assigned to the first inventory cycle. These evaluations therefore do not take into account potential changes in a classification variable characteristic from the earlier to the later inventory (e.g. from private to public ownership).

With the second type of change, the difference in target variables, such as number of stems, growing stock or forest area, is used to assess the change between two inventory cycles. Target variables are generally used to represent states, such as that in NFI4, but can also indicate the net change between any two inventory cycles, e.g. NFI1–NFI4. In evaluations of change using these target variables, the change in a classification variable characteristic is considered in the analysis. Consequently, one can detect, for example, that the forest area without shrub forest increased between the two inventory cycles. This only has an effect on classification variables whose characteristic (class) can actually change, e.g. the affiliation with a forest area or the tree condition.

NFI5
growing stock (stemwood)
diameter at breast height (10 classes) · conifers and broadleaves
regional demarcation: production region
unit: 1000 m³
evaluation area: accessible forest without shrub forest NFI4/NFI5
grid: 1.4 km grid, subgrids 1-5
state 2018/26
		production region
		Jura		Plateau		Pre-Alps		Alps		Southern Alps		Switzerland
diameter at breast height (10 classes)	conifers and broadleaves	1000 m³	±%	1000 m³	±%	1000 m³	±%	1000 m³	±%	1000 m³	±%	1000 m³	±%
n/a	n/a	0	.	0	.	0	.	0	.	0	.	0	.
	conifers	0	.	0	.	0	.	0	.	0	.	0	.
	broadleaves	0	.	0	.	0	.	0	.	0	.	0	.
	indeterminable	0	.	0	.	0	.	0	.	0	.	0	.
	total	0	.	0	.	0	.	0	.	0	.	0	.
12-20 cm	n/a	0	.	0	.	0	.	0	.	0	.	0	.
	conifers	1929	9	2850	10	3069	9	5998	6	957	12	14804	4
	broadleaves	2624	7	2860	7	1938	9	2936	8	3267	8	13626	3
	indeterminable	0	0	0	.	0	0	0	.	0	.	0	0
	total	4554	6	5711	6	5007	6	8935	4	4224	6	28430	3
21-30 cm	n/a	0	.	0	.	0	.	0	.	0	.	0	.
	conifers	4135	8	5302	9	7073	7	12690	5	2430	12	31630	3
	broadleaves	4913	7	4861	7	4868	8	3976	8	5531	8	24150	4
	indeterminable	0	0	0	.	0	.	0	0	0	.	0	0
	total	9048	5	10163	6	11941	5	16666	4	7961	6	55779	2
31-40 cm	n/a	0	.	0	.	0	.	0	.	0	.	0	.
	conifers	6334	8	6349	8	11560	6	18067	4	3955	12	46265	3
	broadleaves	7175	6	6808	7	5679	8	3691	9	4140	9	27493	3
	indeterminable	0	0	0	0	0	.	0	0	0	.	0	0
	total	13508	5	13157	5	17239	5	21758	4	8095	8	73757	2
41-50 cm	n/a	0	.	0	.	0	.	0	.	0	.	0	.
	conifers	7996	7	8618	7	13615	5	22892	4	4308	11	57428	3
	broadleaves	7934	6	7324	7	5753	9	3239	10	2913	10	27163	4
	indeterminable	0	.	0	.	0	.	0	.	0	.	0	.
	total	15929	4	15942	5	19367	4	26131	4	7221	7	84590	2
51-60 cm	n/a	0	.	0	.	0	.	0	.	0	.	0	.
	conifers	7665	7	8406	7	15615	5	20335	4	4353	12	56373	3
	broadleaves	5063	9	8610	7	3983	11	2449	13	1525	15	21630	4
	indeterminable	0	.	0	.	0	.	0	.	0	.	0	.
	total	12728	5	17016	5	19598	5	22784	4	5878	10	78004	2
61-70 cm	n/a	0	.	0	.	0	.	0	.	0	.	0	.
	conifers	4992	11	5982	9	10403	8	12602	6	2351	13	36331	4
	broadleaves	2127	15	4469	10	1458	19	1422	20	813	21	10290	7
	indeterminable	0	.	0	.	0	.	0	.	0	.	0	.
	total	7119	8	10452	6	11861	7	14024	6	3164	10	46621	3
71-80 cm	n/a	0	.	0	.	0	.	0	.	0	.	0	.
	conifers	2632	14	3435	13	5202	11	8414	8	1646	20	21330	5
	broadleaves	766	25	2003	17	853	29	528	28	395	26	4544	11
	indeterminable	0	.	0	.	0	.	0	.	0	.	0	.
	total	3398	12	5438	10	6055	10	8942	8	2040	17	25874	5
81-90 cm	n/a	0	.	0	.	0	.	0	.	0	.	0	.
	conifers	798	30	2090	18	2207	18	3029	14	938	29	9061	9
	broadleaves	114	72	530	34	295	45	160	51	325	32	1423	19
	indeterminable	0	.	0	.	0	.	0	.	0	.	0	.
	total	911	28	2619	16	2501	16	3189	13	1263	23	10484	8
91-100 cm	n/a	0	.	0	.	0	.	0	.	0	.	0	.
	conifers	340	45	294	45	347	41	1556	19	456	39	2994	14
	broadleaves	0	.	544	35	55	100	54	71	145	59	798	27
	indeterminable	0	.	0	.	0	.	0	.	0	.	0	.
	total	340	45	838	27	402	38	1610	18	601	32	3792	12
>100 cm	n/a	0	.	0	.	0	.	0	.	0	.	0	.
	conifers	255	59	155	71	478	41	690	30	314	59	1891	20
	broadleaves	0	.	416	56	0	.	0	.	411	55	827	39
	indeterminable	0	.	0	.	0	.	0	.	0	.	0	.
	total	255	59	571	45	478	41	690	30	725	40	2719	19
total	n/a	0	.	0	.	0	.	0	.	0	.	0	.
	conifers	37076	5	43481	5	69568	4	106274	3	21708	8	278106	2
	broadleaves	30716	4	38426	4	24882	6	18456	6	19464	5	131944	2
	indeterminable	0	0	0	0	0	0	0	0	0	.	0	0
	total	67792	3	81907	3	94450	3	124730	2	41172	4	410050	1

Table citation

Abegg, M.; Ahles, P.; Allgaier Leuch, B.; Cioldi, F.; Didion, M.; Düggelin, C.; Fischer, C.; Herold, A.; Meile, R.; Rohner, B.; Rösler, E.; Speich, S.; Temperli, C.; Traub, B.,
2023: Swiss national forest inventory - Result table No. 1353911. Birmensdorf, Swiss Federal Research Institute WSL

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